Purpose: To investigate the problems of practice experienced by novice school principals as they transition into their new occupation, focusing in particular on the first 3 months on the job--a critical transition period according to the literature. Research Methods/Approach: This theory-building, mixed-methods, longitudinal study examines a random sample of novice principals from one cohort of new Chicago Public School principals. Using interviews at two time points, we systematically examine the "reality shocks" novices encounter as they experience their new occupation firsthand. Findings: A major "reality shock" for novice principals as they transitioned into their new occupation was a sense of ultimate responsibility. This sense of ultimate responsibility contributed to three core problems of practice--task volume, diversity, and unpredictability. While almost all novices experienced the responsibility shock as well as one or more of the practice problems, the conditions of novices' transitions to the principalship either eased or exacerbated the level of practice problems they encountered. Implications for Research and Practice: Our account shows how the volume, diversity, and unpredictability of tasks emerge early and intensify over new principals' first 3 months on the job, largely due to new principals' sense of ultimate responsibility. In our conclusion, we discuss the implications of our work for research and practice by considering ways that problems of practice can be eased for novices as they transition into their new position.