A review is given of the application of in situ transmission electron microscopy to study various processes associated with the crystallization of amorphous thin films. Solid phase epitaxial regrowth of ion-implanted silicon is compared with nucleation and growth in deposited thin films. The mechanism of metal-mediated crystallization is deduced directly from high resolution recordings, and the kinetics of tantalum oxide devitrefication are obtained. The advantages of direct in situ observation are described.