The interactions of biological macromolecules and the flow of regulatory information that controls development, behavior, and homeostasis can be considered a genetic network. The nodes in such networks are genes or their RNA and protein products. The connections are the regulatory and physical interactions among the RNAs, proteins, and cis-regulatory DNA sequences of each gene. Modem molecular genetic techniques have greatly increased the rate at which genes are being recognized and their primary sequences determined. The challenge is to link the genes and their products into functional pathways, circuits, and networks. Analyses of regulatory networks (such as those involving signal transduction and transcriptional regulation cascades) illustrate combinatorial action that implements, for example, digital logic, analog-digital conversions, cross-talk and insulation, and signal integration. Although the existence of sophisticated network elements has been suggested by decades of physiological studies, what is new is the scale and detail becoming available for the components. Much of current molecular biology focuses on identifying new components, defining the regulatory in puts and outputs of each node, and delineating the physiologically relevant pathways. ; © 1995 American Association for the Advancement of Science.