Claims that the outcome of evolution is historically contingent seem to take two different forms. Sometimes, biologists making this claim seem to be saying that the outcome of evolution could have been far different even if the process had started from the same prior conditions, that today’s diversity of organisms is the unpredictable result of chancy evolutionary processes. Other times, they seem to be saying that today’s outcomes are dependent on prior initial conditions, that had these conditions been slightly different, then the evolutionary outcome would have been very different. John Beatty has called the first sense of contingency the “unpredictability version” and the second sense the “causal dependence version”. He claims that these two versions of contingency are different, but not incompatible. In this talk, Waters will propose a way to conceive of historical contingency that combines ideas about unpredictability (or chanciness) and causal dependence. He will use much of the talk to analyze the concept of chanciness. He will argue that the sense of chanciness relevant to evolution is the sense alluded to by Aristotle, not the sense invoked in quantum physics. Waters will appeal to the idea of robustness to clarify how Aristotle’s idea of a chance meeting at the market place relates to the chance outcome of an evolutionary process. He will then show, as Beatty suggested, that the idea that the outcome of evolution are causally sensitive to differences in initial conditions and yet are also chancy in an important sense of the term. Historical contingency involves processes that are both chancy and causally dependent. He will use directed graphs to illustrate what this means.