Cell polarity is a fundamental feature of eukaryotic cells and plays a major role in many developmental processes. The conserved PAR protein system mediates polarization in a wide variety of animal cell types including epithelia, neurons and stem cells. PAR proteins localize asymmetrically within a cell and direct the polarization of downstream cellular processes. Although some in vitro binding interactions between PAR proteins are known, a fundamental unanswered question is how PAR proteins are organized biochemically into a signaling network that can mediate cell polarization. To address this question, we have developed a method to quantify protein-protein interactions at the single-molecule level, and with temporal resolution, in single cells. We are applying this approach to the C. elegans zygote, a single cell that polarizes in a defined manner with known and reproducible timing. We have identified PAR protein complexes that are specifically up- or down-regulated as the zygote polarizes. In particular, our data identify PAR-3 oligomerization as a key regulatory node for controlling cell polarization: PAR-3 forms large oligomers during polarity establishment, but these oligomers are absent in both stably polarized and non-polarized cells. Quantitative live imaging experiments suggest that PAR-3 oligomerization may contribute to polarity establishment by facilitating redistribution of the PAR-3/PAR-6/aPKC complex to the anterior half of the zygote. Ongoing experiments that combine single-cell biochemistry, quantitative live cell imaging, CRISPR/Cas9-induced targeted mutations and computational modeling in this simple system will reveal how regulated interactions between PAR proteins contribute to the establishment and maintenance of cell polarity.