Heterotrimeric G proteins are key players in a transmembrane signalling system that is used by every cell to regulate its basal functions as well as to integrate its specific functions into the whole organism. The complexity of this task is reflected by the diversity of molecular components involved therein and the variety of their potential interactions that have been described thus far. G protein mediated cellular signalling obviously represents a network of interacting pathways that are highly dynamic and are subject to short and long term regulatory processes that adapt the system to changing conditions. The identification of signalling molecules and the description of their functions have provided the foundation for understanding signal transduction processes involving G proteins. Much work, however, is still required to provide an understanding of how these molecular events are orchestrated in time and in space in a living cell.