The anterior-posterior axis of the Caenorhabditis elegans zygote forms shortly after fertilization when the sperm pronucleus and its associated centrosomal asters provide a cue that establishes the anterior-posterior (AP) body axis. In response to this cue, the microfilament cytoskeleton polarizes the distribution of a group of widely conserved, cortically localized regulators called the PAR proteins, which are required for the first mitotic division to be asymmetric. These asymmetries include a posterior displacement of the first mitotic spindle and the differential segregation of cell-fate determinants to the anterior and posterior daughters produced by the first cleavage of the zygote. Here we review recent advances in our understanding of the mechanisms that polarize the one-cell zygote to generate an AP axis of asymmetry.