  The propagation of chaos is a central concept of kinetic theory that serves
to relate the equations of Boltzmann and Vlasov to the dynamics of
many-particle systems. Propagation of chaos means that molecular chaos, i.e.,
the stochastic independence of two random particles in a many-particle system,
persists in time, as the number of particles tends to infinity.
  We establish a necessary and sufficient condition for a family of general
n-particle Markov processes to propagate chaos. This condition is expressed in
terms of the Markov transition functions associated to the n-particle
processes, and it amounts to saying that chaos of random initial states
propagates if it propagates for pure initial states.
  Our proof of this result relies on the weak convergence approach to the study
of chaos due to Sznitman and Tanaka. We assume that the space in which the
particles live is homeomorphic to a complete and separable metric space so that
we may invoke Prohorov's theorem in our proof.
  We also show that, if the particles can be in only finitely many states, then
molecular chaos implies that the specific entropies in the n-particle
distributions converge to the entropy of the limiting single-particle
distribution.
