Zebrafish have been widely used in developmental studies as an excellent in vivo, high-throughput and scalable system for decades, due to the following advantages. Its small size (< 5mm at 7 days post-fertilization (dpf)) and large number of rapidly developing transparent embryos (most organs are developed within 5dpf) enable researchers to undergo whole mount in situ hybridization studies, real-time fate-mapping experiments, and visualizing developing organ morphology. Furthermore, the zebrafish has a well-established genome database that provides information for genomic studies, ranging from microarrays to more advance deep-sequencing studies. Moreover, its highly conserved developmental signalling pathways position it as a good model to correlate with other mammalian studies. In the field of environmental toxicology, zebrafish is used to understand the common and standard toxicology end point, such as phenotypic changes and lethal concentrations (LC50). However, these studies do not fully utilize the features of zebrafish. This chapter aims to provide our experiences and opinion on using zebrafish to understand early developmental defects caused by toxicant exposures. A standard whole-mount in situ hybridization screening protocol was used to monitor three critical early developmental stages (60-75% epiboly, 8-10 somite, and prim-5). The screening provided first-hand information of whether the toxicant affects the dorsoventral (DV) patterning, segmentation, and brain development in zebrafish embryos within 24 hours exposure of toxicants. Since tracing the continuous developmental changes at different stages of mammalian embryos is not easy due to in utero development; the zebrafish provides an excellent alternative way to understand the developmental toxicology of different toxicants.