Cell polarity is a fundamental property of many cells and is required for organismal development. In my PhD thesis I shed light on the molecular mechanisms regulating cell polarity establishment. Using the polarized C. elegans one-cell embryo as a model system, I studied the involvement of Protein Phosphatases (PP) in this process. In the first part of my thesis, I described the possible role of PP1 and PP2A phosphatases in the regulation of the proper localization of the anterior polarity protein PAR-3. In the second part, I found that two PP1 phosphatases antagonize the protein kinase PKC-3, and promote the posterior cortical localization of another polarity protein, PAR-2. Overall, my research describes the crucial role of protein phosphatases for a robust establishment of cell polarity in the C. elegans embryo.