Biological systems are far from thermodynamic equilibrium. Concentration gradients and electrostatic potential differences are the driving forces for diffusive currents and chemical reactions. In this chapter, we present the basic ingredients of nonequilibrium thermodynamics. We derive continuity equations for mass and energy. Entropy production is a bilinear function of the thermodynamic forces which vanish at equilibrium. Close to equilibrium, the fluxes can be approximated as linear functions of the forces and the entropy production as a positive definite symmetric quadratic form. Finally, we discuss stationary states which are characterized by a minimum of entropy production, which is compatible with certain external conditions.