Since stress is a tensor quantity, it is most of the time less well understood than for example forcewhich is a vector. The simple vector calculus used for forces is not valid for stress quantities. To start understanding stress, it is not necessary yet to understand tensor notation. In this chapter, the basic understandings of stress are dealt with without directly introducing the stress tensor; the mathematical concept of stress as a tensor is introduced later in chapter 10. In this chapter, the relationship between principal stresses and normal and shear stress components is derived. Moreover, applications to geological intrusions are outlined and practical field methods for stress measurements are summarized.