This chapter provides an overview of how cells receive, process, and respond to information from the environment, whether the information is in the form of light, smell, or blood-glucose concentration. Signal-transduction cascades mediate the sensing and processing of these stimuli. These molecular circuits detect, amplify, and integrate diverse external signals to generate responses such as changes in enzyme activity, gene expression, or ionchannel activity. This chapter introduces some of the basic principles of signal transduction and important classes of molecules that participate in common signal-transduction pathways.