During the last decennia, our understanding of the neurobiological processes underlying learning and memory has continuously improved, leading to the identification of targets for the development of memory-enhancing drugs. Here we review a class of drugs which has more recently been identified: the phosphodiesterase (PDE) inhibitors. An overview is given of the different PDEs that are known and we focus on three PDEs which have been identified as possible relevant targets for memory improvement: PDE2, PDE4 and PDE5. PDEs differ in the substrate, i.e. cyclic adenosine monophosphate (cAMP) and/or cyclic guanosine monophosphate (cGMP), being hydrolyzed. Since these cyclic nucleotides have been suggested to play distinct roles in processes of memory, selective PDE inhibitors preventing the breakdown of cAMP and/or cGMP could improve memory. The present data suggest that PDE4 (cAMP) is involved in acquisition processes, although a possible role in late consolidation processes cannot be excluded. PDE5 (cGMP) is involved in early consolidation processes. Since PDE2 inhibition affects both cAMP and cGMP, PDE2 inhibitors may improve both memory processes. The field of PDEs is highly dynamic and new isoforms of PDEs are still being described. This may lead to the discovery and development of new memory enhancing drugs that selectively inhibit such isoforms. Such drugs may exert their effects only in specific brain areas and hence possess an improved side effect profile.