Most cells have a complex internal structure of biological rods, ropes and sheets. In terms of the metaphors for the cell in Chapter 1 – a hot air balloon, a sailing ship – our task in Parts I and II of this text was to analyse the individual structural elements of the cell – the masts, rigging and hull – without regard to their interconnection. In Part III, we begin to assemble the units together to form composite systems. As the introduction to Part III, Chapter 7 treats the equilibrium shapes of the simplest systems, namely cells or vesicles without a nucleus or space-filling cytoskeleton. A thorough treatment of complex cells is beyond the reach of our analytical tools; rather, we mention a selection of their properties in Chapters 7–9, before returning to some general attributes of cells in Chapter 10. Many eucaryotic cells live communally as part of a larger organism, and Chapter 8 investigates the interaction between cell membranes, including entropic effects from thermal fluctuations in cell shape. In Chapter 9, we describe mechanisms that cells have developed for changing their shape, as part of cell division or locomotion, for example. The book concludes by returning to the broad perspective established in Chapter 1 and examines some generic features of cell structure, including synthetic cells, and speculates on the mechanical characteristics of the largest and smallest cells.