Phase transition between liquid and solid is a ubiquitous phenomenon in nature and has been used widely in scientific and industrial communities. Most metals have high melting points, posing a challenge to studying and using their phase transition-enabled abrupt changes in properties. Gallium and its alloys with low melting points (termed as liquid metal) can easily switch between solid and liquid in response to thermal energy at room temperature, accompanied by a significant variation in properties, including thermal enthalpy, stiffness, shape and volume, surface adhesion force, and catalytic activity, providing a rather straightforward and effective platform to observe the phase transition phenomena of metals and leverage switchable properties. The overwhelming majority of these studies and applications have quickly emerged in the last 10 years, which is at the beginning of the journey of applying the phase transition of liquid metal. This article is dedicated to presenting a systematic review of the fundamentals and latest progress of phase transition science and engineering of liquid metal and proposing future research directions and the main challenges that lie ahead. It is expected to help mold the category of such cutting-edge subjects and inspire more research and applications in the near future.