There is growing interest among the travel behavior community in activity scheduling and rescheduling behavior. This paper reports an analysis of activity modification behavior using extensive data for activity scheduling and rescheduling collected from a recent survey. Models that are developed describe the frequency and type of schedule modification. The analysis indicates that activity location and coscheduling with peers were much less frequently modified than activity timing and duration and that the degree of spatial flexibility of an activity plays an important role. The implications of these findings both for the modeling of scheduling and rescheduling behavior and for the development of scheduling support tools are discussed.