In what follows, I suggest that it makes good sense to think of the truth of (at least some of) the probabilistic generalizations made in the life sciences as metaphysically grounded in stochastic mechanisms in the world. To further understand these stochastic mechanisms, I take the general characterization of mechanism offered by MDC (Philos Sci 76(1):1–25, 2000) and explore how it fits with several of the going philosophical accounts of chance: subjectivism, frequentism, Lewisian best-systems, and propensity. I argue that neither subjectivism, frequentism, nor a best-system-style interpretation of chance will give us what we need from an account of stochastic mechanism, but some version of propensity theory can. I then draw a few important lessons from recent propensity interpretations of fitness in order to present a novel propensity interpretation of stochastic mechanism according to which stochastic mechanisms are thought to have probabilistic propensities to produce certain outcomes over others. This understanding of stochastic mechanism, once fully fleshed-out, provides the benefits of (1) allowing the stochasticity of a particular mechanism to be an objective property in the world, a property investigable by science, (2) a way of quantifying the stochasticity of a particular mechanism, and (3) a way to avoid a problematic commitment to the causal efficacy of propensities (and dispositional properties in general).