Cyclic nucleotides such as cGMP and cAMP play pivotal roles as second messengers in many biological processes. Upon stimulation of appropriate signal transduction pathways, the levels of these messengers change rapidly. Such variations in second messenger level may also be spatially restricted within the cell. To detect dynamic and local changes in second messengers, we need to study them in living cells with high spatial and temporal resolution. Focusing on cAMP, here we describe how imaging of an EPAC-based FRET sensor in single cells provides that spatiotemporal resolution.