Epithelial cells are patterned not only along their apical–basolateral axis, but also along the plane of the epithelial sheet; the latter event is regulated by the planar cell polarity (PCP) pathway. PCP regulates diverse outputs, such as the distal placement of a hair in all cells of the Drosophila wing, and convergent extension movements during gastrulation in the vertebrate embryo. This primer describes the molecular mechanisms that initiate and establish PCP, as well as biochemical pathways that translate PCP signaling to cell type-specific patterning events. The primer concludes with a discussion of current topics in the field with two PCP researchers, Matt Kelley, Ph.D., and Helen McNeill, Ph.D. Developmental Dynamics 233:695–700, 2005. © 2005 Wiley-Liss, Inc.