Charm Mixing - Theory

We discuss Standard Model (SM) and New Physics (NP) descriptions of D0 mixing. The SM part of the discussion addresses both quark level and hadron level contributions. The NP part describes our recent works on the rate difference Delta Gamma_D and the mass difference Delta M_D. In particular, we describe how the recent experimental determination of Delta M_D is found to place tightened restrictions on parameter spaces for 17 of 21 NP models considered in a recent paper by Hewett, Pakvasa, Petrov and myself.