The demand for deformable optoelectronic devices that are bendable, stretchable, and foldable has significantly increased. Accordingly, recent research has focused on ensuring the flexibility and stretchability of devices under various deformation modes, making the evaluation and enhancement of the mechanical reliability crucial. Here, we review strategies to improve the flexibility and stretchability of deformable optoelectronic devices, including intrinsic approaches to increasing the elastic deformation limit of materials and extrinsic structural design strategies at the device level. We introduce recent advances in the mechanical reliability of these devices in different deformation modes. Furthermore, we discuss mechanical testing methods for enhancing mechanical reliability, including conventional bulk-scale mechanical testing, nanomechanical tests for constituent thin-layer materials, and computational simulation tools for analysis and prediction.