  We give an account of some results, both old and new, about any $n\times n$
Markov matrix that is embeddable in a one-parameter Markov semigroup. These
include the fact that its eigenvalues must lie in a certain region in the unit
ball. We prove that a well-known procedure for approximating a non-embeddable
Markov matrix by an embeddable one is optimal in a certain sense.
