Whom should I ask to write letters of recommendation?

The most useful letters come from those instructors who know you and your academic work well. In general, the election committees prefer letters from members of the faculty, but letters from teaching fellows or course assistants are also acceptable. Seniors who are writing theses are strongly advised to have their faculty adviser write on their behalf. The election committees generally find letters from faculty within one's department or committee of concentration most useful, but will also accept letters from faculty outside of a student's concentration.