  This is a survey paper about reciprocal processes. The bridges of a Markov
process are also Markov. But an arbitrary mixture of these bridges fails to be
Markov in general. However, it still enjoys the interesting properties of a
reciprocal process. The structures of Markov and reciprocal processes are
recalled with emphasis on their time-symmetries. A review of the main
properties of the reciprocal processes is presented. Our measure-theoretical
approach allows for a unified treatment of the diffusion and jump processes.
Abstract results are illustrated by several examples and counter-examples.
