I describe the astronomical regions in which chemistry is known to occur, and list the types of molecules that have been positively identified. Some of the successes and failures of chemistries that have been proposed to give rise to these molecular species are reviewed. The evidence which supports the view that processes on and in interstellar dust contribute to the overall chemistry are highlighted. Present ideas concerning the nature of interstellar dust and its response to the changing conditions in the interstellar medium are presented, so that the role of dust in interstellar chemistry can be considered. Current problems and directions of future work are indicated.