In 1995, Watts Humphrey introduced the Personal Software Process in his book, A Discipline for Software Engineering (Addison Wesley Longman, Reading, Mass.). Programmers who use the PSP gather measurements related to their own work products and the process by which they were developed, then use these measures to drive changes to their development behavior. The PSP focuses on defect reduction and estimation improvement as the two primary goals of personal process improvement. Through individual collection and analysis of personal data, the PSP shows how individuals can implement empirically guided software process improvement. The full PSP curriculum leads practitioners through a sequence of seven personal processes. The first and most simple PSP process, PSPO, requires that practitioners track time and defect data using a Time Recording Log and Defect Recording Log, then fill out a detailed Project Summary Report. Later processes become more complicated, introducing size and time estimation, scheduling, and quality management practices such as defect density prediction and cost-of-quality analyses. After almost three years of teaching and using the PSP, we have experienced its educational benefits. As researchers, however, we have also uncovered evidence of certain limitations. We believe that awareness of these limitations can help improve appropriate adoption and evaluation of the method by industrial and academic practitioners.