Tissue folding plays an important role in animal morphogenesis. To date, the folding of epithelial sheets has mostly been attributed to mechanical force generation at the apical sides of epithelial cells. Now, Liyuan Sui and Christian Dahmann demonstrate that mechanical tension at lateral cell sides contributes to tissue folding in the Drosophila wing disc epithelium. The authors use optogenetic activation of RhoGEF2 to provide spatial control of mechanical tension, first showing that this approach stimulates both F-actin polymerization and Myosin II motor activity to increase actomyosin contractility. Following on from this, the authors systematically test whether force generation at apical, basal or lateral sides of wing disc cells is sufficient for fold formation. Their results reveal that, although increased apical tension is inefficient for fold formation, optogenetic activation of RhoGEF2 at lateral cell sides is sufficient to trigger the formation of deep folds. They further report that the repeated activation of RhoGEF2 allows for longer lasting, but reversible, fold formation. These findings highlight the importance of an increase in lateral tension for epithelial folding and suggest that different epithelia can employ diverse mechanisms to form a fold.