Sociologists are increasingly attentive to the mechanisms responsible for cause-and-effect relationships in the social world. But an aspect of mechanistic causality has not been sufficiently considered. It is well recognized that most phenomena of interest to social science result from multiple mechanisms operating in sequence. However, causal chains—sequentially linked mechanisms and their enabling background conditions—vary not just substantively, by the kind of causal work they do, but also structurally, by their formal properties. In this article, the author examines the nature (...) of causal chains, identifies major structural dimensions along which they differ, and makes a case that a mechanism-based explanation would be enhanced if causal chains and their structures were brought to the analytical forefront. (shrink)