While “determinism” is a standard technical term in philosophy, theology, and physics, the disciplines generally do not agree on its meaning. Philosophers and theologians often employ a notion of physical determinism that was introduced in the seventeenth century, pointing to classical mechanics as a paradigm case. Physicists, in contrast, use a mathematical definition based on the underlying differential equations. This latter approach opens new possibilities for divine action that does not depend on exotic physics. This is an important consideration given the rise deterministic interpretations of quantum mechanics. The chapter ends with a critique of the causal closure of nature.