Cyclic AMP (cAMP) is a key second messenger in all cells. It is compartmentalized within cells and its levels are controlled, as a result of spatially discrete signaling cassettes controlling its generation, detection and degradation. Underpinning compartmentalized cAMP signaling are ∼20 members of the phosphodiesterase-4 (PDE4) family. The selective inhibition of this family generates profound, functional effects and PDE4 inhibitors are currently under development to provide potential, novel therapeutics for the treatment of inflammatory diseases, such as asthma, chronic obstructive pulmonary disease and psoriasis, as well as treating depression and serving as cognitive enhancers. Here, we delineate the range of PDE4 isoforms, their role in signaling, their structural biology and related preclinical and clinical pharmacology.