Abstract: Crystal freezing and melting have been studied for more than a century, but a theory about their fundamental mechanisms remains unavailable. Experimentally it is difficult to observe the motions of single molecules inside a crystal. In the last two decades, micrometer-sized colloidal particles have been used as outstanding model systems for phase-transition studies. The initial nucleation process can be directly observed under a microscope, and single-particle trajectories can be tracked by image processing. This approach provides rich microscopic information on melting and crystallization.