This book presents a general classical field theory, incorporating continuum mechanics, electrodynamics, and thermodynamics. The continuum equations of material behavior are derived from the principles of Onsager's non-equilibrium thermodynamics supplemented with dynamic degrees of freedom. The book contains the basic principles and methods of modern continuum mechanics and of rheology. Non-equilibrium thermodynamics is discussed in detail. Applications include elasticity, thermoelasticity, viscoelasticity, plasticity, rheooptics, etc. The models of rheology are developed within a consistent thermodynamic framework. Viscoelastic and plastic response, Ostwald's curve of generalized Newtonian fluids, creep, elasticity preceding plastic flow, the rules of rheooptics, etc., are discussed, and the empirical Cox-Merz rule is proved. The thermodynamic results are compared to the results of microscopic theories. Several kinds of colloids, polymers, and liquid crystals are studied. The technical level of the book is high. It is designed for engineers, physicists, natural scientists and applied mathematicians.