Chapter 2 provides a review of the fundamentals of Earth Science needed to understand the plate tectonic model. It provides an overview of the different kinds of forces involved in geoscience and continues on to stress, deformation and strain rate. The chapter explores the global stress pattern of the uppermost crust, shows how such information is retrieved, and discusses how it relates to the plate tectonic model. In this context, deformation structures such as faults, fabrics, folds and shear zones are briefly reviewed. Rheology, which relates to how the different parts of our planet react to stress, is also discussed. Further, simple models for rheological variations or profiles through the outer part of the planet are discussed, as such profiles strongly influence how lithospheric plates deform.