The goals of modeling diffusion bonding can be regarded as twofold: to optimize the selection of the process variables for a given material and to provide an understanding of the mechanisms by which bonding is achieved. This article describes the existing models of diffusion bonding with an assumption that the surfaces to be joined are free of contaminants and oxide, that bonding occurs between similar materials, and that the materials are single-phase metals. It discusses the mechanisms considered for diffusion bonding and limitations of existing models.