Phase models are popular in the history of technology and in innovation research, as they allow case studies to be categorized into different development phases. However, these categorizations usually remain descriptive and therefore somewhat arbitrary, e.g., life-cycle models, used in the history of science and technology and in the sociology of technology. Only a few approaches pursue an analytical claim, namely, to uncover the social mechanisms that play a role in the transition from one phase to the next, e.g., a cycle model of technological change that combines ideas from evolutionary economics and sociology. Similarly, a phase model views innovation as the result of a multi-stage process of social construction of technology driven by changing networks of actors. Recently, transition research has taken up the idea of phase transitions and developed concepts of how this knowledge can be used practically to promote sustainable transitions. To summarize, this chapter provides an overview of prominent phase models and discusses their strengths and weaknesses.