G proteins play a critical role in signal transduction across cell membranes. Information about the diversity of G protein structure and function has provided valuable insights into the nature of the complex actions exerted by hormones and neurotransmitters on different cells and biological systems. A common finding of the biophysical and biochemical studies described above is that G proteins have the potential to couple neurotransmitter or hormone receptors to multiple cellular effector systems. When this occurs in a cell, it may allow a particular hormone or transmitter to regulate a variety of different cellular events simultaneously. Furthermore, it has become clear that different receptors can couple to the same G protein. When this occurs in the same cell, it may provide the basis for the convergent regulation of cell activity by various hormones or neurotransmitters. Thus, G proteins greatly diversify the manner by which hormones and neurotransmitters can regulate cells. As more information is available on the mechanisms by which G proteins recognize and interact with receptors and effector systems, we may be able to better understand the specific events involved in signal transduction and the subtle processes by which hormones and neurotransmitters can control cell activity.