This textbook treats solids and fluids in a balanced manner, using thermodynamic restrictions on the relation between applied forces and material responses. This unified approach can be appreciated by engineers, physicists, and applied mathematicians with some background in engineering mechanics. It has many examples and about 150 exercises for students to practise. The higher mathematics needed for a complete understanding is provided in the early chapters. This subject is essential for engineers involved in experimental or numerical modelling of material behaviour.