The polarised architecture of epithelial cells is crucial for tissue development and integrity. Variations in plasma membrane lipid composition are involved in epithelial polarity, and the apical domain contains primarily PtdIns(4,5)P2, whereas the basolateral membrane is characterised by high levels of PtdIns(3,4,5)P3. But how is this difference maintained? On page 3393, Patrick Laprise and colleagues provide an answer by showing that the transmembrane protein Crumbs, the small GTPase Rac1 and PI3K fine-tune the membrane lipid composition in polarised epithelial cells. They show that in the Drosophila embryo, active PI3K results in the activation of Rac1 and that this, in turn, enhances PI3K activity. Furthermore, they find that Crumbs regulates polarity by interfering with this positive-feedback loop. By limiting the activity of PI3K and Rac1, Crumbs has an important role in limiting the production of PtdIns(3,4,5)P3 in the apical membrane. The researchers also show that overexpression of constitutively active Rac1 results in decreased Crumbs expression and a highly restricted apical domain. Thus the system is additionally fine-tuned by the inhibition of Crumbs activity in the apical domain by the Rac1–PI3K module. These data shed light on the mechanisms involved in establishing epithelial polarity and highlight that these signals need to be carefully balanced to obtain the correct cellular arrangement.