As stress, strain is a tensor quantity. After having introduced the stress tensor in previous chapter, the strain tensor is introduced in this chapter. The chapter begins with an introduction into the various measures  of strain. Principle stretches, which are a powerfull tool to describe simple strain patterns, are then descriped after which also the strain tensor, needed for the description of more complex systems, is dealt with. With having the prior knowledge from this first three paragraphs, the deformation theory can be introduced as well as the visualization of the variation in the orientation of the principal strain axes. The invariants of the strain tensor summarized in the sixth paragraph. Knowing about both the stress tensor, from previous chapter, and the strain tensor, the relationship between the two is exemplified. The chapter is concluded with a description of the more general deformation tensor.