The Picture Exchange Communication System (PECS) is a popular augmentative and alternative communication intervention for individuals with developmental disabilities that includes six, sequential phases of instruction. We systematically reviewed published and unpublished single-case PECS studies for details about the percentage of participants mastering PECS phases, how long it took participants to master PECS phases (in trials to mastery), and what constituted mastery of PECS phases. We found most participants mastered only the first two phases of PECS, and Phases I, II, and IV showed the slowest and widest variability in trials to mastery. Moreover, participants who were taught the third PECS phase learned to select from an average array of three symbols, which may limit the range of messages they can communicate. We discuss our findings in light of a few important limitations and provide directions for future researchers along with recommendations for professionals who might consider PECS.