This chapter explores the evolution of complex multicellularity and its implications for organismal agency, with a focus on sessile multicellular lineages. Although multicellularity has evolved independently several times, only a few lineages have reached high levels of organizational complexity. The chapter analyzes the structural and functional differences between sessile and motile integrated multicellular organisms, highlighting the significance of cell walls in non-motile multicellular life and the evolution of specialized tissues in plants and fungi. It examines the constraints faced by sessile organisms and their adaptive strategies, particularly their reliance on secretory interactions and plastic growth responses to environmental stimuli. Ultimately, the chapter demonstrates that an increase in structural complexity does not necessarily correspond to a parallel increase in the complexity of agency.