Most science students readily understand that following fertilization, a single-celled egg must undergo multiple rounds of cell division to become a multicellular organism. This transformation is so universal among animal embryos that developmental biologists refer to the process with a single term: ''gastrulation.'' During gastrulation, many if not all of the newly cleaved cells dramatically change their location within the embryo in a variety of different ways, sometimes individually and sometimes as contiguous sheets of cells. With the widespread use of fluorescence microscopy and such fluorescent probes as green fluorescent protein (GFP), a renaissance in the study of cellular movements during gastrulation in living embryos began in the latter part of the twentieth century. These studies have been extensively documented in a recent monograph (www. gastrulation.org) that describes the molecular interactions that cause and, in turn, result from gastrulation in embryos from many different animal taxa (Stern, 2004). The author uses this article to review several videos on gastrulation, which are published as freely accessible supplements to this monograph.