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The evolution of multicellular life from unicellular ancestral types involves a hierarchical shift in the level at which selection operates. The shift, from cells to collectives, depends on the emergence of Darwinian properties at the level of nascent collectives. However, from the very earliest phases—even before the emergence of higher-level Darwinian properties—the stage is set for the evolution of conflict. Here we consider the range of ways by which cooperation and conflict manifest at different levels of biological organization. We give prominence to the emerging idea that conflict is a central driver in the evolution of biological complexity and, in particular, that solutions to conflict, notably those that arise from selection operating at different temporal scales, have fueled the evolution of individuality.