There is nearly universal agreement that coercion is an evil. Even when it is necessary to avoid a greater evil or to attain some good, it is still a necessary evil. There is also nearly universal agreement that, other things being equal, one ought not to exercise coercion. Here the agreement ends. There is little agreement about just when coercion is justified. More surprisingly, there is little agreement about what coercion is. This latter controversy is more fundamental, and this paper is an attempt to shed some light on it.