Students' reactions to classroom learning and the mastery of science vary along a wide spectrum of attitudes and emotions. In particular, we argue here that how learners encounter and learn subject matter is a function of their level of cognitive development. We describe the stages of cognitive development based on the work of William Perry and demonstrate their relevance to the undergraduate science classroom. With examples drawn from biochemistry, we attempt to show that, depending on the student's developmental level, there will be different abilities to handle the range of assignments and activities s/he can expect to experience in the average classroom. The college science instructor can benefit from knowledge of these stages and can work through their implications to develop strategies and techniques to regulate collective student learning.