  The invariant measure is a fundamental object in the theory of Markov
processes. In finite dimensions a Markov process is defined by transition rates
of the corresponding stochastic matrix. The Markov tree theorem provides an
explicit representation of the invariant measure of a stochastic matrix. In
this note, we given a simple and purely combinatorial proof of the Markov tree
theorem. In the symmetric case of detailed balance, the statement and the proof
simplifies even more.
