Caged compounds are light-sensitive probes that functionally encapsulate biomolecules in an inactive form. Irradiation liberates the trapped molecule, permitting targeted perturbation of a biological process. Uncaging technology and fluorescence microscopy are 'optically orthogonal': the former allows control, and the latter, observation of cellular function. Used in conjunction with other technologies (for example, patch clamp and/or genetics), the light beam becomes a uniquely powerful tool to stimulate a selected biological target in space or time. Here I describe important examples of widely used caged compounds, their design features and synthesis, as well as practical details of how to use them with living cells.