Cell migration is a crucial process for animal development and physiology. Many if not all cell types move at a specific developmental time or in specific situations. Their migration places, shapes, or repairs the tissue of which they are part. In cancer, cell migration is also responsible for metastasis. For the past decades, the cellular and molecular bases of migration have been thoroughly analyzed in vitro. However migration in vivo appears much more complex, migration being largely influenced by the complex cellular environment (neighboring cells, extracellular matrix, chemo-attractants, mechanical constraints…). Cell movements in the organism are thus much less understood. Gastrulation is the first step in development when cells move in the embryo, to set up the future body plan. It is a highly conserved morphogenetic process transforming a seemingly unstructured blastula into a highly organized gastrula-stage embryo composed of the three germ layers ectoderm, mesoderm and endoderm. In addition to its crucial role in embryogenesis, gastrulation is an ideal system to study cell migration since it combines high quality imaging, genetic tools and direct manipulation (transplants, explants, ablations, application/relief of mechanical constraints…), thus allowing for multi-disciplinary approaches. We are using the fish gastrula as a model system to analyze cell migration in vivo, focusing mainly on endoderm and mesoderm migration.