Clickers are a popular tool in large science classes. The authors find that clickers can also be used in small undergraduate- and graduate-level science classes, and to some extent also in laboratory classes, to achieve the same purposes as in large classes. Issues that can be addressed using clickers include fully engaging all students, identifying misconceptions, assessing preparedness, gauging the understanding of concepts, eliciting discussion, and identifying students who need additional help. However, much still depends on effective usage of the clickers, which in turn depends on time and effort invested by the instructor in developing effective questions and in working these questions into the class flow in a seamless manner. (Contains 1 table.)