We argue that the transition from unicellular to multicellular systems raises important conceptual challenges for understanding agency. We compare several MC systems displaying different forms of collective behavior, and we analyze whether these actions can be considered organismically integrated and attributable to the whole. We distinguish between a ‘constitutive’ and an ‘interactive’ dimension of organizational complexity, and we argue that MC agency requires a radical entanglement between the related processes which we call “the constitutive-interactive closure principle”. We explain in detail (...) that this is not possible without a regulatory center functionally integrating the two dimensions, and we also argue that, in turn, this type of regulation would not be possible without a special type of organization between the cells required for the development and maintenance of systems capable of integrated behavior. (shrink)