Philosophers of science principally associate the causal-mechanical view of scientific explanation with the work of Peter Railton and Wesley Salmon. In this paper I shall argue that the defects of this view arise from an inadequate analysis of the concept of mechanism. I contrast Salmon's account of mechanisms in terms of the causal-nexus with my own account of mechanisms, in which mechanisms are viewed as complex systems. After describing these two concepts of mechanistic explanation, I show how the complex-systems approach to mechanism avoids certain objections that can be raised against Salmon's version of the causal-mechanical approach. I conclude by discussing how mechanistic explanations in the complex systems approach can provide understanding by unification.