One of the unifying themes of modern biology is evolution. In introductory biology courses, evolution is studied in some detail and used as a focal point for many courses. For many teachers of those courses, the greatest frustration is their inability to help students become Darwinian. Students do not reject evolutionary ideas, but they quickly slip into a Lamarckian approach to explain the changes in organisms over time. Their misconceptions seem so ingrained that most experiences have little effect on them. This article presents the different kinds of misconceptions and suggests ways to correct them.