This chapter discusses the law of balance of mass, as well as the laws of balance of forces and moments. The important related concept of stress is this then presented as formalized by Cauchy in terms of a central theorem of Continuum Mechanics, which asserts that satisfaction of global balance of forces and moments is equivalent to the existence of a symmetric tensor field in the deformed body called the Cauchy stress, such that the traction vector acting across each oriented surface element at a point in the body is given by the Cauchy stress tensor operating linearly on the outward unit normal to the surface at that point. In addition, the stress tensor must satisfy a partial differential equation, known as the equation of motion, which asserts that the divergence of the stress tensor plus a body force per unit volume, is equal to the mass density times the acceleration.