Modularity has become a central and remarkably useful concept in evolutionary developmental biology, offering an explanation of how independent, interacting units make possible developmental events and evolutionary changes. These modules exist at several different levels of organization, from genes to signal transduction pathways to cell populations. Cell populations, which are multicellular modules, provide an opportunity both to clarify our notion of modularity and to reexamine such central concepts as cell-to-cell communication. Rosine Chandebois’s work on “cell sociology” is reframed in the language of modularity in order to show how groups of cells maintain their collective identity during development and how signaling information from the group’s environment is received by the constituent cells. In a “test case” for our account of multicellular modules, we consider the vertebrate neural crest, a migratory cell population made up of several subpopulations. The neural crest proves yet again to be a complex case, showing the importance of taking into account both developmental and evolutionary transformations of multicellular modules.