This chapter discusses the optical probes for cyclic 3',5'-adenosine monophosphate (cAMP), which was the first molecule to be explicitly recognized as a “second messenger,” a mediator between extracellular stimuli and intracellular biochemical responses. Many roles have been assigned to cAMP. However, one area where cAMP research has been lagging is the detailed exploration of the spatial and temporal dynamics of cAMP signaling. Optical probes for Ca2+ have revealed that Ca2+ signaling is often remarkably complex in fine structures. The chapter discusses various methods for measuring cAMP—namely, assay in cell lysates, immunocytochemistry, fluorescent analogues of cyclic nucleotides, and fluorescently labeled cAMP receptor protein from E. coli. The chapter also describes a major sample application, which is the visualization of the spread of cAMP within MA-10 Leydig tumor cells from sites of relatively localized hormone stimulation.