We have conducted in-situ transmission electron microscopy (TEM) experiments on thinned single crystal barium titanate in order to study the effects of applied electric field, temperature, electron beam irradiation and UV irradiation on domain nucleation and propagation. We observe two basic modes of domain wall motion; (i) a lateral motion which uniformly widens or narrows the total domain width; (ii) a “zipping” motion in which one end of a domain narrows to a point, which then propagates lengthwise, widening (or narrowing) the domain behind it. Both domain creation and destruction can occur by this latter process. When cooling from above Tc, domain growth usually occurs by the “zipping” motion. We believe that both the lateral and “zipping” modes of motion are related. The “zipping” mode tends to occur in the presence of inhomogeneous long-range strain fields, or when trapped charges are present locally. In some instances, the trapped charge is strong enough to show significant image contrast in bright-field. Domain motion, initiated by heat, electric fields or UV irradiation, moves such charges. A model of domain motion is presented which shows how displacement charge can be injected into the ferroelectric, and which may contribute to the fatigue of these materials.