, Edit: updated August 22, 2020
I receive many requests from my students for letters of recommendation. Writing letters of recommendation is a part of my job, and one that I take seriously. It is my job to help my students succeed. Students who think they are likely to need letters of recommendation in the future have a responsibility to cultivate relationships with faculty which will enable the faculty to write letters for them. As a general rule, most faculty will write letters for students who have been successful working with the faculty across a long time frame. Getting an A in a large lecture class is probably not enough for a faculty to feel comfortable writing you a letter.
As a general rule, I will write letters for any graduate student whose committee I have been on and/or who has worked for me as an RA or TA and performed well, any postdoc I have mentored (or other post-PhD level colleague who I know well), any research assistant who has worked under my direct supervision and/or with my close collaboration, and any undergraduate who has worked for me or conducted research under my direct supervision (e.g. as an honors student). I will also write letters for students who took multiple classes with me and received an A in all of those classes. I occasionally will write letters for students who do not meet all of these criteria, but usually only under unusual circumstances (for example, a student who overcame significant obstacles to perform well in my classes, or who improved their performance significantly over the course of the time they studied with me, a student who I came to know well outside of the classroom for other reasons such as advising, etc.) The reason I do not write letters for students I do not know well is because I cannot write them strong or supportive letters. For students this means that it is your responsibility to cultivate relationships with faculty who can serve as mentors, and simply doing ok in their classes is probably insufficient to achieve this.
I can respond most effectively if you provide me 2 or more weeks notice prior to my having to submit the letter. With less notice I may not be able to provide the letter (and more notice is always better - I'm occasionally in the field or consumed by other duties and unable to process recommendation letters even in 2 weeks). If I have sent a similar letter recently on your behalf (e.g. you are applying to graduate school and just applied to a similar graduate program) it will be much easier for me to send a second letter, and I may need less lead time (though again, I am sometimes away from my computer for a week or 2 at a time, particularly over the summer and during standard holiday periods). Please do not hesitate to ask for several letters - writing one letter takes serious work, but subsequent similar letters are much less work. I am aware that many students are hesitant to ask for "too many" letters, but from my perspective, the big ask is to write one letter, and each additional similar letter is not a whole lot of work.
When you request a letter from me, please provide me detailed information about what you are applying for, along with as much of your application information as possible (for example, a draft of application essays or cover letters, a CV or resume). If not obviously included in your application, provide me with an explanation of why you want the job (or to get into that particular graduate program, etc.) Please describe how you know me - i.e. if you took 2 of my classes two years ago, mention which classes you took which semesters, if you were my advisee mention which year you graduated or plan to graduate. Include details of your performance in my classes (i.e. I worked on a project about X topic and got an A). For better or worse, I teach a couple hundred students a year and even students I knew well a couple years ago sometimes fade in my memory to be replaced by the latest student, so I do need this information to write you a letter. You should provide me this information for me *every* time you request my letter AND *every* time you use my name as a reference (even if a letter isn't initially required). If I have already written several letters for you within the last 3-4 months, I'm more likely to be able to respond to a request for a letter on a short time frame, as I can usually repurpose a recent letter.
Note that if you are are a graduate student applying for an academic job, there is a quirk in many university HR systems that means you may be asking me for letters without realizing it. Specifically, many university HR application websites will ask you for "the names and contact information for three references" or something along these lines, implying that they will only ask for references from short-listed candidates. However once you enter this information, a request for a letter of reference is sometimes sent to me automatically. This request often contains no information about what position you are applying for. So please tell me when you are listing my name as a reference, even if you don't think I will be contacted. And please provide me with details about these applications too!
If you want me to write a strong letter for you, please read Leonard Cassuto's wise advice - all of which applies to me as well as to him. Please include this information. This advice applies each time you request a letter, unless the letters are very similar or are in very close succession - if you sent me all of this information when you requested a letter a couple of months ago, send it again this time too. This will help me personalize your letter, which will make it more effective.
In general, I do not provide letters to students to submit with their applications, nor do I provide "generic" letters which students can keep on file to be used on an as-needed basis. In nearly all cases that I am aware of, employers, graduate schools, and fellowships strongly prefer or require that letter writers submit unique letters themselves, and provide faculty instructions for how to submit their letters directly to the reviewers. Please provide me with this information. If there is an unusual situation which does not conform with these normal practices, please inform me and provide detailed information and documentation.
Finally, if you only know me from taking one of my larger lecture classes, keep in mind that I probably do not know you very well, and you are almost certainly better off asking for a letter from a professor who knows you better, and will thus write a more detailed and personalized letter. Letters I write for students who I only know because they got an A in a class with more than 100 students are of necessity brief and formulaic . This kind of letter is not very effective support (worse if I only know you because you got below an A in one of these classes, in which case I really don't feel comfortable providing a letter). If you want strong letters of recommendation it is imperative that you get to know professors by finding a way to work closely with them. I would like to help everyone out but I have found that weak letters from student I only know from large classes simply do not help those students.
I receive many requests from my students for letters of recommendation. Writing letters of recommendation is a part of my job, and one that I take seriously. It is my job to help my students succeed. Students who think they are likely to need letters of recommendation in the future have a responsibility to cultivate relationships with faculty which will enable the faculty to write letters for them. As a general rule, most faculty will write letters for students who have been successful working with the faculty across a long time frame. Getting an A in a large lecture class is probably not enough for a faculty to feel comfortable writing you a letter.
As a general rule, I will write letters for any graduate student whose committee I have been on and/or who has worked for me as an RA or TA and performed well, any postdoc I have mentored (or other post-PhD level colleague who I know well), any research assistant who has worked under my direct supervision and/or with my close collaboration, and any undergraduate who has worked for me or conducted research under my direct supervision (e.g. as an honors student). I will also write letters for students who took multiple classes with me and received an A in all of those classes. I occasionally will write letters for students who do not meet all of these criteria, but usually only under unusual circumstances (for example, a student who overcame significant obstacles to perform well in my classes, or who improved their performance significantly over the course of the time they studied with me, a student who I came to know well outside of the classroom for other reasons such as advising, etc.) The reason I do not write letters for students I do not know well is because I cannot write them strong or supportive letters. For students this means that it is your responsibility to cultivate relationships with faculty who can serve as mentors, and simply doing ok in their classes is probably insufficient to achieve this.
I can respond most effectively if you provide me 2 or more weeks notice prior to my having to submit the letter. With less notice I may not be able to provide the letter (and more notice is always better - I'm occasionally in the field or consumed by other duties and unable to process recommendation letters even in 2 weeks). If I have sent a similar letter recently on your behalf (e.g. you are applying to graduate school and just applied to a similar graduate program) it will be much easier for me to send a second letter, and I may need less lead time (though again, I am sometimes away from my computer for a week or 2 at a time, particularly over the summer and during standard holiday periods). Please do not hesitate to ask for several letters - writing one letter takes serious work, but subsequent similar letters are much less work. I am aware that many students are hesitant to ask for "too many" letters, but from my perspective, the big ask is to write one letter, and each additional similar letter is not a whole lot of work.
When you request a letter from me, please provide me detailed information about what you are applying for, along with as much of your application information as possible (for example, a draft of application essays or cover letters, a CV or resume). If not obviously included in your application, provide me with an explanation of why you want the job (or to get into that particular graduate program, etc.) Please describe how you know me - i.e. if you took 2 of my classes two years ago, mention which classes you took which semesters, if you were my advisee mention which year you graduated or plan to graduate. Include details of your performance in my classes (i.e. I worked on a project about X topic and got an A). For better or worse, I teach a couple hundred students a year and even students I knew well a couple years ago sometimes fade in my memory to be replaced by the latest student, so I do need this information to write you a letter. You should provide me this information for me *every* time you request my letter AND *every* time you use my name as a reference (even if a letter isn't initially required). If I have already written several letters for you within the last 3-4 months, I'm more likely to be able to respond to a request for a letter on a short time frame, as I can usually repurpose a recent letter.
Note that if you are are a graduate student applying for an academic job, there is a quirk in many university HR systems that means you may be asking me for letters without realizing it. Specifically, many university HR application websites will ask you for "the names and contact information for three references" or something along these lines, implying that they will only ask for references from short-listed candidates. However once you enter this information, a request for a letter of reference is sometimes sent to me automatically. This request often contains no information about what position you are applying for. So please tell me when you are listing my name as a reference, even if you don't think I will be contacted. And please provide me with details about these applications too!
If you want me to write a strong letter for you, please read Leonard Cassuto's wise advice - all of which applies to me as well as to him. Please include this information. This advice applies each time you request a letter, unless the letters are very similar or are in very close succession - if you sent me all of this information when you requested a letter a couple of months ago, send it again this time too. This will help me personalize your letter, which will make it more effective.
In general, I do not provide letters to students to submit with their applications, nor do I provide "generic" letters which students can keep on file to be used on an as-needed basis. In nearly all cases that I am aware of, employers, graduate schools, and fellowships strongly prefer or require that letter writers submit unique letters themselves, and provide faculty instructions for how to submit their letters directly to the reviewers. Please provide me with this information. If there is an unusual situation which does not conform with these normal practices, please inform me and provide detailed information and documentation.
Finally, if you only know me from taking one of my larger lecture classes, keep in mind that I probably do not know you very well, and you are almost certainly better off asking for a letter from a professor who knows you better, and will thus write a more detailed and personalized letter. Letters I write for students who I only know because they got an A in a class with more than 100 students are of necessity brief and formulaic . This kind of letter is not very effective support (worse if I only know you because you got below an A in one of these classes, in which case I really don't feel comfortable providing a letter). If you want strong letters of recommendation it is imperative that you get to know professors by finding a way to work closely with them. I would like to help everyone out but I have found that weak letters from student I only know from large classes simply do not help those students.