We provide a conceptual model of the supply and demand forces influencing the exit behavior of teachers and employ this model to examine the exit behavior of highly effective teachers in New York City.  We estimate these relationships employing extensive administrative data on New York City teachers and their students and an exit survey of over 5000 New York City teachers.   We examine a variety of self-reported reasons leavers provide for their decisions and examine their employment after leaving New York City.  Our intent is to distinguish the forces that may cause highly effective teachers to leave teaching to determine if policies or practices could deter them.  We examine the schools where principals appear to be more successful in encouraging less effective teachers to leave while retaining relatively more effective teachers and the characteristics that distinguish these principals and schools from those who are less successful in this differential retention.