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This paper investigates whether peer effects play an important role in retirement savings decisions. We use individual data from the staff of a university to study whether individual decisions to enroll in a Tax Deferred Account plan sponsored by the university (and the choice of the mutual fund vendor for people who choose to enroll) are affected by the decisions of other employees in the same department. To overcome the identification problems, we separate the departments into sub-groups (along gender, status, age, and tenure lines) and we instrument the average participation of each peer group by the salary or tenure structure in this group. Our results suggest that peer effects are important. We find significant owngroup peer effect on participation and on vendor's choice, but no cross-group peer effects. (JEL D83, 122) *MIT, Department of Economics E52-252g, 50 Memorial Drive, Cambridge, MA 02142, [email protected]. Harvard University, Department of Economics, Littauer Center, Cambridge, MA 02138, [email protected]. We thank Abhijit Banerjee, David Cutler, Jonathan Gruber, Lawrence Katz, Michael Kremer, Philip Lima, Brigitte Madrian, Sendhil Mullainathan, Kaivan Munshi, Linda Nolan, James Poterba, and Rosemary Rudnicki for helpful disf ussions, and Kirsten Carter, Regina Perris, and Polly Price for giving us access to the data.