Cells in multicellular organisms communicate through a large variety of extracellular chemical signals. This chapter examines some of the most important mechanisms by which cells send signals and interpret the signals they receive. It presents an overview of the general principles of cell signaling. The chapter considers two of the main systems animal cells use to receive and interpret signals, followed by a brief discussion of cell signaling mechanisms in plants. It also considers how extensive and intricate signaling networks interact to control complex behaviors. The chapter concerns signal reception and transduction—the events that cell biologists have in mind when they refer to cell signaling. It looks at the different types of extracellular signals that cells send to one another. Different intracellular signaling pathways interact, enabling each cell type to produce the appropriate response to a combination of extracellular signals.