This chapter explores the crucial role of diffusion in shaping the properties of crystalline solids like steel, silicon, and germanium. These materials play vital roles in engineering and electronics, and their properties are highly dependent on their atomic structure and grain morphology. Notably, their surface layers are susceptible to changes during heat treatment due to diffusion of interstitial atoms at high temperatures. Diffusion, a mass transport phenomenon driven by atomic motion, plays a key role in altering the atomic arrangement. The chapter focuses on the concept of diffusion through random walk theory, a statistical model that helps understand the movement of atoms within a solid, liquid, and gas. Fick’s laws of diffusion mathematically describe this process. The chapter categorizes diffusion into two main mechanisms: (1) interstitial diffusion, where small atoms like carbon navigate through the lattice structure of iron, and (2) substitutional diffusion, where larger atoms like phosphorus replace host atoms in a silicon crystal. This chapter explores analytical techniques for solving diffusion problems with a focus on concentration-independent diffusion coefficients. Through illustrative examples, the chapter prepares readers with the mathematical tools to solve diffusion problems and understand their influence on the properties of crystalline materials.