In the early mammalian embryo cells undergo differentiation into specific lineages, which are assembled into a blueprint for the mature body plan. The coordinated positioning of each tissue involves extensive cell movement and tissue rearrangement, which is ultimately controlled by cytoskeletal remodeling at a cellular level. Examples of two critical early events are migration of the Anterior Visceral Endoderm (AVE) and gastrulation. Both involve epithelial cells undergoing rearrangement and directed cell movement, yet the underlying molecular mechanisms that control such events remain undefined. We have been investigating a possible role for the Planar Cell Polarity (PCP) pathway, an evolutionarily conserved pathway, which in Drosophila regulates tissue polarity and cell movement primarily through modification of the cytoskeleton. A core group of PCP proteins have been defined including the membrane receptor Frizzled, the cytoplasmic proteins Dishevelled and Prickle, and the transmembrane proteins Flamingo and Strabismus. They co-localize to form asymmetric complexes within the cell, which modulate cytoskeletal components thus resulting in changes in cell shape or position or arrangement of a whole tissue. Specifically we have been investigating the Prickle orthologues Prickle! and 2 and Test in, and the Flamingo orthologues Celsrl-3. Initially we defined the early embryonic expression patterns of these genes and as a consequence of their expression in both the AVE and gastrulating cells we chose to analyze the function of Celsrl and Testin. We used genetic approaches to analyze their function. We generated two mouse lines, one carrying a Cre inducible Dominant-Negative form of Celsrl in the Rosa26 locus and another with a genetrap in the Testin locus resulting in a null allele. In addition to the characterization of the embryonic expression patterns of the Prickle and Flamingo orthologues this thesis describes the generation and analysis of both the Celsrl and Testin mutant lines and the subsequent analysis of both relating to a proposed role for PCP in the early mammalian embryo.