This chapter introduces the concept tensors by defining the original tensor, i.e. Cauchy’s stress tensor. However, first some basic principles of dynamics related to continuous materials: solids, fluids, and gases, have to be presented. Dynamics, which is the science of motion of bodies with mass and the forces that cause this motion, is often subdivided into Kinematics and Kinetics. Kinematics is the geometry of motion with velocity and acceleration as the most important concepts.