This chapter presents the phase behaviour and morphology of block copolymer melts. The main techniques for investigating block copolymer microstructures are transmission electron microscopy (TEM) and small-angle x-ray or neutron scattering. Phase diagram for a conformationally symmetric diblock copolymer, calculated using self-consistent mean field theory, along with illustrations of the equilibrium morphologies is presented. Some systems, such as the poly(styrene-butadiene-methylmethacrylate) (PS-PB-PMMA) triblock copolymers, exhibit a particularly rich complexity in phase behaviour, forming multiple ordered phases as listed. Schematic of several morphologies observed for ABC triblock copolymer melts are shown.