We will discuss the proof of the weight part of Serre's conjecture for rank two unitary groups in the unramified case (that is, the Buzzard-Diamond-Jarvis conjecture for unitary groups). This is joint work with Toby Gee and Tong Liu. More precisely, we prove that any Serre weight which occurs is a predicted weight; this completes the analysis begun by Barnet-Lamb, Gee, and Geraghty, who proved that all predicted weights occur. Our methods are purely local, using Liu's theory of (phi,G-hat) modules to determine the possible reductions mod p of certain two-dimensional crystalline Galois representations.