For multicellular organisms to form and operate, cellular behavior must be vastly more complex than what is seen at the single-cell level. Not only must cells regulate their own growth and behavior, but they must also communicate and interact with their neighbors to ensure the correct behavior of the entire organism. Intercellular communication occurs in a variety of ways, ranging from hormonal communication on the level of the entire body to localized interactions between individual cells. The discussion in this chapter is (mostly) limited to cellular communication processes that occur between cells or over a region of a small number of cells. Other forms of communication and control, such as hormone feedback systems, are described in other chapters.