This paper begins by developing a causal theory of mechanisms in medicine, and illustrates the theory with the example of the mechanism of the disease anthrax as elucidated by Koch. The causal approach to mechanisms is then compared to the Machamer, Darden, Craver (MDC) approach. At first sight the two approaches appear to be very different, but it is argued that the divergence is less than it initially seems. There are some differences, however, and it is argued that, where these differences occur, the causal approach to mechanisms is superior.