The physical mechanisms responsible for the annealing of ion implanted semiconductors for a wide range of materials and annealing conditions have been identified through the use of in-situ dynamical measurements of optical properties. These measurements combined with post-anneal sample characterization, i.e., channeling, electrical measurements, etc., lead to the identification of two different annealing mechanisms. For the case of Q-switched lasers a liquid phase epitaxial regrowth of the damaged layer is indicated. For c.w. lasers solid phase epitaxial regrowth can occur.