This paper describes the concept of ``switch'' in the study of combinatorial structures including codes, designs, and related structures. Switching is a certain local transformation that does not alter the basic parameters of a combinatorial structure. For example, a switch of a binary code is a transformation that concerns exactly one coordinate and keeps the studied parameter of the code (for instance, minimum distance) unchanged. This paper gives a precise definition for a switch for codes, designs, and related structures. Then it unifies earlier results including a new result for covering codes, and applies switching exhaustively to some common structures with small parameters.