We investigate the molecular mechanisms by which cells produce and detect chemotactic signals and translate this information in directed coordinated movement up or down chemical gradients in the social amoebae Dictyostelium discoideum, and during gastrulation in the chick embryo. In Dictyostelium starvation for food induces the aggregation of up to hundreds of thousands of individual amoebae into a multi-cellular aggregate. During aggregation the cells differentiate into several distinct celltypes, which sort out to form a migrating slug, which after a variable period of migration transforms into a fruiting body consisting of a stalk supporting a mass of spores. Experiments show that chemotactic cell migration in all stages of development is controlled by propagating waves of the chemo-attractant cAMP. At present we concentrate on investigation of the mechanisms that drive chemotactic cell sorting which results in slug formation. We use quantitative imaging techniques to investigate cell type specific differences in signal transduction dynamics, polarised activation of the actin-myosin cytoskeleton and force generation that drive cell sorting. Gastrulation in the chick embryo starts with the occurring of extensive cell flows that result in the formation of the primitive streak, a structure through which the mesoderm and endoderm precursor cells ingress to take up their correct positions in the embryo. We are tracking the in-vivo migration of these cells during streak formation and after their ingression through the primitive streak to gain insight in the mechanisms that drive these cell movements. Our current hypothesis is that formation of the primitive streak as well as the movement of the mesoderm cells after their ingression through the streak is controlled by a combination of attractive and repulsive guidance cues, delivered at least in part by members of the FGF, VEGF, PDGF and Wnt families of signalling molecules.