This chapter introduces rock mechanics, a field within geomechanics that studies how rocks react to mechanical forces. It is divided into engineering and geological rock mechanics, essential in civil, mining, petroleum, and environmental engineering. The focus is on how forces affect rocks, including velocity, displacement, deformation, and fracture under static or dynamic loads. Key concepts include stress (force per unit area) and strain (deformation per unit length), with normal stress acting perpendicular and shear stress parallel to surfaces. Rock strength is evaluated through tests, with deformation categorized as elastic (reversible), plastic (irreversible), or fractural (breaking under stress). Elastic modulus (Young’s modulus) measures stiffness, and Poisson’s ratio indicates the ratio of lateral to axial strain. Rocks exhibit varied material properties: elastic materials return to shape post-stress, plastic materials deform without breaking, and viscous materials resist flow. Rocks can be homogeneous or heterogeneous in composition, and isotropic or anisotropic in properties. Density, influenced by minerals and porosity, affects weathering resistance, with denser rocks like basalt more resistant than less dense pumice. This chapter forms a basis for understanding rock mechanical behavior, crucial for engineering and geological applications.