In this chapter, we focus specifically on clonal multicellularity, where multicellular groups form through daughter cells remaining attached to mother cells after division. This type of group formation has evolved in several prominent multicellular taxa, including the animals, plants and fungi, and is unique in that it guarantees clonal relatedness between cells. This is in contrast to aggregative group formation, found in the cellular slime molds, some algae, ciliates and many species of bacteria, where genetically distinct cells come together to form the multicellular body (see Chapters 5-8). Clonal relatedness allows highly cooperative behavior to evolve, including altruism, and is the only type of group formation that has led to the evolution of obligate multicellularity. We give an introduction to clonal multicellularity and set the scene with a short primer in social evolution theory. We then review several examples of species with clonal group formation and assess the benefits and costs of multicellular cooperation in clonal groups. We conclude by discussing the impact of the environment on multicellular group formation and asking whether it is possible to evolve obligate multicellularity without clonality.