The understanding of development sessions, the phases during which a developer actively modifies a software system, is a valuable asset for program comprehension, since the sessions directly impact the current state and future evolution of a software system. Such information is usually lost by state-of-the-art versioning systems, because of the checkin/checkout model they rely on: a developer must explicitly commit his changes to the repository. Since this happens in arbitrary and sometimes long intervals, recovering the changes between two commits is difficult and inaccurate, and recovering the order of the changes is impossible. We have implemented an evolution monitoring prototype which records every semantic change performed on a system, and is able to completely reconstruct development sessions. In this paper we use this fine-grained information to understand and characterize the development sessions as they were carried out on two object-oriented systems.