Coordinated cell movements and reciprocal tissue interactions direct the formation of the definitive germ layers and the elaboration of the major axes of the mouse embryo. Genetic and embryological studies have defined the major molecular pathways that mediate these morphogenetic processes and provided 'snapshots' of the morphogenetic program. However, it is increasingly clear that this foundation needs to be validated, and can be significantly refined and extended using live imaging approaches. In situ visualization of these processes in living specimens is a major goal, as it provides unprecedented detail of the individual cellular behaviors, which translate into the large-scale tissue rearrangements that shape the embryo.