Zebrafish represent a promising vertebrate model system for the study of triplicated genes that contribute to the pathophysiology of Down syndrome. Their optical transparency, external fertilization, fecundity, and genetic accessibility make them ideal for determining how increased expression of orthologs of Hsa21 genes perturbs embryonic development. Trisomy of human chromosome 21 (Hsa21) alters the levels of hundreds of genes that can have critical developmental roles, leading to pleiotropic effects on the body. In this chapter, we describe the conservation of many organ systems affected in individuals with Down syndrome (e.g., skeletal, cardiovascular, immune, and more) and the tools available in zebrafish to assess their morphology and function. Although a limitation of zebrafish is that aneuploidy cannot be achieved with current genome-engineering technology, studying many individual Hsa21 orthologs and their combinations is immediately accessible. Looking to the future, multiple compounds found using zebrafish have entered the clinic or clinical trials. Establishing zebrafish models of relevant phenotypes will lay the necessary groundwork for large-scale drug screens to discover treatments for aspects of Down syndrome.