Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. The time evolution of core impurity toroidal rotation velocity profiles has been measured with a tangentially viewing crystal x-ray spectrometer array. Following the L-mode to EDA (enhanced D α ) H-mode transition in both Ohmic and ICRF heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a time scale of order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. The ensuing steady state toroidal rotation velocity profiles in EDA H-mode plasmas are relatively flat, with V φ ∼ 50 km/s, and the momentum transport can be simulated with a simple diffusion model. Assuming the L-H transition produces an instantaneous edge source of toroidal torque (which disappears at the H-to L-mode transition), the momentum transport may be characterized by a diffusivity, with values of ∼ 0.07 m 2 /s during EDA H-mode and ∼ 0.2 m 2 /s in L-mode. These values are large compared to the calculated neoclassical momentum diffusivities, which are of order 0.003 m 2 /s. Velocity profiles of ELM-free H-mode plasmas are centrally peaked (with V φ (0) exceeding 100 km/s in some cases), which suggests the workings of an inward momentum pinch; the observed profiles are consistent with simulations including an edge inward convection velocity of ∼ 10 m/s. In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the center, indicating the presence of a negative radial electric field well in the vicinity of the barrier foot. Upper single null diverted and inner wall limited L-mode discharges exhibit strong counter-current rotation (with V φ (0) ∼ −60 km/s in some cases), which may be related to the observed higher Hmode power threshold in these configurations. For plasmas with locked modes, the toroidal rotation is observed to stop (V φ ≤5 km/s). 1