Capacities assign a weight, measure of importance, or value to each subset with respect to the given reference set. This allows one to model interaction, taking into account any redundancies or complementary effects between variables, but comes at the expense of increased complexity in the number of parameters that need to be defined and interpreted. Considerable effort has gone into addressing this, resulting in some well-established theory and indices that allow the behaviour of capacities to be well understood. Many of these indices serve as alternative representations that allow for new construction methods and simplifications. This chapter presents the most well-known indices and their properties.