Most cancers end up with the death of patients caused by the formation of secondary tumors, called metastases. However, how these secondary tumors appear and develop is only poorly understood. A fine understanding of the multiple steps of the metastasis cascade requires in vivo models allowing high spatiotemporal analysis of the behavior of metastatic cells. Zebrafish embryos combine several advantages such as transparency, small size, stereotyped anatomy, and easy handling, making it a very powerful model for cell and cancer biology, and in vivo imaging analysis. In the following chapter, we describe a complete procedure allowing in vivo imaging methods, at high throughput and spatiotemporal resolution, to assess the behavior of circulating tumor cells (CTCs) in an experimental metastasis assay. This protocol provides access, for the first time, to the earliest steps of tumor cell seeding during metastasis formation.