The book develops an account of objective chances as properties of events (e.g. of coin tosses) defined by the degrees of belief (e.g. that the coin will land heads) which embody knowledge of those chances. The chances manifest propensities: dispositional properties of objects (e.g. a coin's bias). The account is applied to theories of radioactive decay and of how human death risk increases with age, and is used to show how knowledge of propensities underlies apparently a priori derivations of classical chance distributions. Finally it shows how propensities entail indeterminism and are consistent with a Humean view of laws of nature.