Stress is a force per unit area with the mathematical properties of a tensor quantity. In chapter 3 and 4, the ellipsoid method to represent principal stresses is already explained. For the description of more complex stress field, tensor notation is much more powerful. Therefore, this chapter will first explain the limitations of the ellipsoid method and after that will start with an introduction to tensor notation and mathematics. Normal and shear stresses are then described on an arbitrary plane. The different elements within the stress tensor are also treated one by one, and the importance of the right coordinate system is explained. Manipulation of the stress tensor can then be done by applying complementary mathematical concepts, which are introduced in paragraphs 6 to 9. The chapter concludes with an outline of the special case of plane stress in 2D and a summary of the significance of the stress tensor invariants.