Abstract: Micrometer-sized colloidal particles in suspension can self-assemble into various structures and exhibit rich phase behaviors as atoms. Phase transitions can be driven by tuning the particles' size, shape, interaction or number density. In particular, colloidal particles can be directly imaged and their thermal-motion trajectories can be tracked under optical video microscopy, which provides powerful model systems to study the microscopic kinetics of phase transitions. Many possible studies on phase transition are opened, due to the great advances in synthesizing colloids with tunable interaction, nonspherical shape and activity, and in self-assembly and computer simulation techniques in recent years. Here, we summarize the recent studies on phase transitions in colloidal model systems, with an emphasis on crystallization, melting and solid-solid transition. We also briefly point out the challenges and future directions in this field.