Understanding the molecular organisation of intracellular signalling pathways is a topic of considerable research interest. Since many signalling enzymes are widely distributed and have several substrates, a critical component in signal transduction is the control of specificity. This is achieved, in part by the assembly of multiprotein complexes where clusters of signalling enzymes create focal points to disseminate the intracellular action of many hormones. This is particularly true for the cAMP dependent protein kinase (PKA) that is localised throughout the cell via its association with A-kinase anchoring proteins (AKAPs). Recent data suggest that some AKAPs also interact with phosphodiesterases (PDEs). Compartmentalisation of PDEs not only provides an elegant means to control PKA activation by monitoring the local cAMP flux, but also serves to concentrate and segregate the action of these important regulatory enzymes.