This paper reviews the literature on the modulation of cyclic adenosine 3',5'-monophosphates (cAMP) by inhibition of phosphodiesterase 4 (PDE 4) enzyme. It highlights why PDE 4 is an interesting anti-inflammatory target, particularly for asthma. Focus is given to the selective and non-selective phosphodiesterase inhibitors presently disclosed in the current scientific literature and patent literature up to June of 1996. It reviews the pre-clinical and clinical data available for these inhibitors and discusses the future for development of novel PDE 4 inhibitors.