Abstract-We analyze the impact of mutual coupling on the capacity of MIMO systems with compact antenna arrays. Existing studies present conflicting views on the effect of mutual coupling. This is, in part, due to their different scopes and underlying assumptions of the system setups. In this paper, we aim to give a comprehensive picture by first examining the impact of mutual coupling on three capacity-related performance measures: antenna correlation, efficiency and bandwidth. While the first two aspects have received significant attention, antenna bandwidth with mutual coupling is a relatively uncharted territory. We show that while implementing a good matching network can drastically improve the system capacity for narrowband systems in the presence of strong mutual coupling, the same conclusions may not necessarily apply to wideband cases. To exemplify this, we carry out capacity simulations for an end-to-end MIMO system, where a recently proposed S-parameter approach is used in conjunction with the 3GPP-3GPP2 channel model to model realistic wideband channel and antenna effects at both transmit and receive ends.