When a solid body undergoes external loading, it manifests deformation through both geometric and volumetric changes. This chapter presents a rigorous mathematical framework for characterizing deformation in solid continua, systematically examining the fundamental principles governing these transformations. The development of strain measures is initiated through an intuitive approach, beginning with the uniaxial stretch of a prismatic rod, which serves as a foundational case study applicable to both finite and infinitesimal deformation regimes. Building upon this elementary configuration, the general three-dimensional strain measures for finite deformations are subsequently developed through logical extension and mathematical generalization. Within this comprehensive framework, both small deformation and small strain cases are systematically derived as special instances of the finite deformation theory, maintaining their intrinsic relationship with the general formulation.