My introduction to developmental psychology took place about 12 years ago while teaching science in California. I recall being astounded at reading that some children think that changing the shape of a piece of clay changes its weight and that some children even believe a change in shape changes the amount of clay. Since that time I have had the opportunity to investigate what these sorts of student misconceptions mean to the science teacher. In this chapter I would like to raise a number of questions and discuss a series of investigations undertaken to answer these questions to understand better the nature of intellectual development and what it means to the science teacher. At the conclusion I would hope that you will have a reasonably clear notion of how developmental psychology can be applied to improve instruction in the science classroom as well as an awareness of some important unresolved issues.