  In this paper we present a combinatorial proof of Selberg's integral formula.
We start by giving a bijective proof of a Theorem about the number of
topological orders of a certain related directed graph. Selberg's Integral
Formula then follows by induction. This solves a problem posed by R. Stanley in
2008. Our proof is based on Andersons analytic proof of the formula. As part of
the proof we show a further generalisation of the generalised Vandermonde
determinant.
