The theory of evolution has transformed biology from a largely descriptive science to a causal one. However, few psychologists have a thorough understanding of evolution. As a result, psychologists tend not to consider evolutionary explanations for the phenomena they investigate, or they present flawed evolutionary arguments that violate fundamental principles of evolutionary theory. The primary objective of this article is to outline the basic principles of evolution as they relate to the study of behaviour in general. The article then goes on to demonstrate the application of evolutionary theory to the study of human behaviour in particular, and concludes with a discussion of problems that can occur when evolutionary theory is applied incorrectly.