All the processes involved in growth and metabolism of cells require an input ofenergy. In living cells,'production, transport, conversion and utilization of energy'are fundamental processes that are facilitated via metabolic pathways involving a large number of tightly regulated enzyme-catalysed reactions. The regulation ofan enzyme system can be accomplished in a number of different ways, eg by modulation of its substrate concen-trations, by specific regulatory molecules, by post-translational modifications or by subcellular compartmentation. An example of the latter type is creatine kinase (CK), a key enzyme ofcellular energetics, representing an enzyme system with a number of isoenzymes, which are in part compartmentalized specifically at those places where energy is' produced'or'utilized'. Recent experimental data suggest that CK is located near the sites where the'real action'occurs, eg where force generation by motor proteins, ion-transport across membranes by ion pumps and other ATP-dependent processes take place. The creatine kinase/phosphocreatine (CK/PCr) system seems to play a complex, multi-faceted role in cellular energy homeostasis.