This chapter begins with a description of the structural properties of the antipassive, including case, agreement, verb transitivity, and the antipassive morpheme, focusing on languages of the Americas. Given that the antipassive is an alternate form for a transitive ergative sentence describing the same event, a key question is: What determines the choice of one structure over the other? The answer to this question is explored from two perspectives: functional and theoretical. A brief discussion of the antipassive in non-ergative languages follows.