This is a survey on the global theory of constant mean curvature surfaces in Riemannian homogeneous 3-manifolds.These ambient 3-manifolds include the eight canonical Thurston 3-dimensional geometries, i.e.R 3 , H 3 , S 3 , H 2 × R, S 2 × R, the Heisenberg space Nil3, the universal cover of PSL2(R) and the Lie group Sol3.We will focus on the problems of classifying compact CMC surfaces and entire CMC graphs in these spaces.A collection of important open problems of the theory is also presented.