The playful statement written by Josiah Gibbs, who claimed identity as both a mathematician and physicist, provides some truth in jest. Mathematical treatments carry an elegance and beauty that can be appreciated within the abstract world of pure contemplation. However, descriptions of physical processes must be anchored within the allowable states and transitions defined by thermodynamics. As we begin to focus on the specific processes that drive biosphere-atmosphere fluxes we will return to the concept of flux that was established in the last chapter, but now we will construct a physiochemical foundation beneath that concept. Fluxes of scalars and vectors are driven by states of thermodynamic disequilibrium. A flux of mass or energy represents work that is done at the expense of internal energy that is derived from a state of thermodynamic disequilibrium. Thus, as we open this chapter we will develop the thermodynamic context for energy and work, and we will discuss their roles as underlying drivers of biogeochemical fluxes. This will require us to spend some time on the fundamental laws of thermodynamics, the concept of equilibrium, and the various forms of energy that drive biogeochemical processes. One of the concepts we will develop in some detail is the biogeochemical context of potential energy. Potential energy is how we define the capacity for components in a natural system to do work. Potential energy exists in a thermodynamic system that is in a state of disequilibrium; in contrast, a system that is at equilibrium lacks potential energy, and therefore lacks the capacity to generate work on its surroundings or on other systems.