 The importance of constraints for understanding cellular and other biological phenomena has been developed recently by Hooker (2011b, 2013) and Moreno and Mossio (2015). Hooker makes clear that, although the term constraint suggests limitations, constraints also extend possibilities—to offer a cellular example, microtubules that run from the cell center to the periphery restrict the movement of the kinesin and dynein motors that move on them but also provide a possibility for transport of organelles to distant locations. Moreno and Mossio in particular develop a perspective that links the focus on constraints to the idea that living cells are autopoietic. Drawing upon Maturana and Varela’s (1980) conception of living systems as autopoietic machines—machines that provide a network of production that enables the construction of the living system—Moreno and Mossio add a focus on the thermodynamic requirements of cells. Cells, as highly organized systems, are far from equilibrium and require a continual source of free energy to carry out the operations required to synthesize new components and resist the tendency towards equilibrium. What constraints do in organisms, on their account, is direct flows of free energy to perform the work of building, repairing, and reproducing the organism.