Atypical protein kinase C (aPKC) is a central component of the evolutionarily conserved Par3-Par6-aPKC complex, one of the fundamental regulators of cell polarity. We recently demonstrated that these proteins interact with Neph-nephrin molecules at the slit diaphragm of the glomerular filtration barrier. Here, we report that podocyte-specific deletion of aPKCλ/ι in mice results in severe proteinuria, nephrotic syndrome, and death at 4 to 5 wk after birth. Podocyte foot processes of knockout mice developed structural defects, including mislocalization of the slit diaphragm. In the glomerulus, aPKCλ/ι was primarily expressed in developing glomerular epithelial cells and podocyte foot processes. Interestingly, under physiologic conditions, aPKCλ/ι translocated from the apical surface to the basolateral side of developing podocytes, and this translocation preceded the development of foot processes and formation of slit diaphragms. Supporting a critical role for aPKCλ/ι in the maintenance of slit diaphragms and podocyte foot processes, aPKCλ/ι associated with the Neph-nephrin slit diaphragm complex and localized to the tips of filopodia and leading edges of cultured podocytes. These results suggest that aPKC signaling is fundamental to glomerular maintenance and development.