A theory for the evolution of a plasma current in toroidal magnetic configurations follows from considering the plasma to be made of current fibers. The current fiber elements replace the central role of the magnetic field lines of the traditional theory. A set of simple rules determines the behavior of the plasma from energy constraints. The concept of electromechanical oscillations leads to an improved understanding of dynamic plasma behavior. Fiber theory predicts experimental observations of dense Z pinches, spheromaks, and reversed-field pinches. Some characteristic tokamak phenomena are analyzed in terms of the fiber theory.