The basic idea of mechanistic theories of causality is that A causes B if there is a mechanism M linking A to B. One of the principal advocates of this view was Wesley Salmon. Salmon developed several versions of the mechanistic theory of causality. He began, by trying to define causal processes in terms of the transmission of marks, an approach, which had originally been suggested by Reichenbach in 1928. This chapter describes the Dowe–Salmon theory of causality. The basic criticism of the Dowe–Salmon theory is that it lays too much emphasis on physics and so does not apply to causality in other areas such as medicine. The causal theory of mechanisms differs from the Machamer, Darden and Craver approach in that it uses the general concept of cause throughout, while they prefer to focus on activities which are more specific causal verbs.