There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin‐mountable, and wearable strain sensors are needed for several potential applications including personalized health‐monitoring, human motion detection, human‐machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin‐mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Transition metal dichalcogenides (TMDs) layers of molecular thickness, in particular molybdenum disulfide (MoS 2 ), become increasingly important as active elements for mechanically flexible/stretchable electronics owing to their relatively high carrier mobility, wide bandgap, and mechanical flexibility. Although the superior electronic properties of TMD transistors are usually integrated into rigid silicon wafers or glass substrates, the achievement of similar device performance on flexible substrates remains quite a challenge. The present work successfully addresses this challenge by a novel process architecture consisting of a solution‐based polyimide (PI) flexible substrate in which laser‐welded silver nanowires are embedded, a hybrid organic/inorganic gate insulator, and multilayers of MoS 2 . Transistors fabricated according to this process scheme have decent properties: a field‐effect‐mobility as high as 141 cm 2 V −1 s −1 and an I on / I off ratio as high as 5 × 10 5 . Furthermore, no apparent degradation in the device properties is observed under systematic cyclic bending tests with bending radii of 10 and 5 mm. Overall electrical and mechanical results provide potentially important applications in the fabrication of versatile areas of flexible integrated circuitry.