The conformally invariant measure on self-avoiding loops

We show that there exists a unique (up to multiplication by constants) and natural measure on simple loops in the plane and on each Riemann surface, such that the measure is conformally invariant and also invariant under restriction (i.e. the measure on a Riemann surface S' that is contained in another Riemann surface S, is just the measure on S restricted to those loops that stay in S'). We study some of its properties and consequences concerning outer boundaries of critical percolation clusters and Brownian loops.