Molecular motors are nano- or colloidal machines that keep the living cell in a highly ordered, stationary state far from equilibrium. This self-organized order is sustained by the energy transduction of the motors, which couple exergonic or 'downhill' processes to endergonic or 'uphill' processes. A particularly interesting case is provided by the chemomechanical coupling of cytoskeletal motors which use the chemical energy released during ATP hydrolysis in order to generate mechanical forces and to perform mechanical work. We describe a general network theory for molecular motors which leads to local and nonlocal balance conditions that are valid far from equilibrium and generalize the well-known detailed balance conditions for equilibrium states. The nonlocal balance conditions may also be viewed as generalizations of the classical law of mass action to small systems that transduce chemical energy into mechanical work. As a pedagogical example, we discuss the simple case of a single motor head. We also review the application to two-headed motors such as kinesin, which serves as a paradigmatic example for energy transduction via chemomechanical coupling.