Optically dark or nonradiative excitons with large wave vector exceeding the wave vector of light play important role in the dynamics of excitons and polaritons in semiconductor heterostructures. The interaction of bright and dark excitons plays particularly important role in formation of polaritons and their dynamics in structures with microcavities. In high quality structures, the reservoir excitons can persist for a long time (tens of nanoseconds) and they can be accumulated up to large densities.