Diffusion is an omnipresent, but the important phenomenon in a wide variety of disciplines and applications in physical, chemical, biological, geologic, materials science and engineering processes. While diffusion-controlled phase transformations involve in a variety of materials processes, ranging from solidification to different solid-state transformations. Modeling of diffusion and diffusion-controlled phase transformations in alloys has been of long-standing fundamental interests because advance modeling can help to improve the understanding of complex materials processes. Moreover, as the recent boost in Integrated Computational Materials Engineering (ICME) and Materials Genome Initiative (MGI) projects, additional emphasis on the necessity and urgency of the quantitative modeling of diffusion and diffusion-controlled phase transformations in alloys has been laying, which can provide useful information for accelerating the novel alloys design. Consequently, the purpose of this book, entitled "Modeling of Diffusion and Diffusion-Controlled Phase Transformations in Alloys" is to provide a collection of the commonly used computational approaches for modeling diffusion and diffusion-controlled phase transformations, as well as their current status, recent developments, future trends and applications in different alloys.