To study the mechanisms regulating heart size and shape, we take advantage of the unique arsenal of experimental approaches available in zebrafish. A large part of the appeal of the zebrafish derives from the transparency of its embryo, which permits high-resolution inspection of heart size, shape, and function. Additionally, the zebrafish has proven to be excellent for largescale genetic screens and subsequent identification of mutated genes. Screening for cardiac phenotypes is particularly effective, since the zebrafish embryo does not require a functional cardiovascular system for its survival during embryogenesis. By combining the benefits of zebrafish genetics and embryology, we can identify crucial regulators of chamber formation and determine their precise impacts on cell fate and cell behavior.