Educators at all levels are concerned with the development of their students' critical thinking skills. But how can we reliably measure any change in this ability, and how long does it take? This book summarizes the results of a two-year investigation of cognitive development in college students. Student performance on exams at different levels of Bloom's Taxonomy was used to estimate improvement in higher-level thinking. The California Critical Thinking Skills Test (CCTST) was used as an independent and discipline-neutral estimate of this ability. Data analysis confirmed the cumulative hierarchical relationship of Bloom's, with students averaging 73% on exam questions at the knowledge level and 53% at the application and analysis levels, although statistically significant improvement in critical thinking by students was not detected over two semesters. Clearly, critical thinking is difficult for students and takes longer to develop than we might have guessed. The findings of this study have implications for curricular decision-making and course management, and are relevant at an institutional and an individual course level.