This review aims to provide a timely survey at the intersection of skin-mountable strain sensors and printed electronics. The review is organized as follows: Sensing mechanisms employed in printed sensors are introduced in Section 2. In Section 3, various printing techniques and printed flexible and stretchable strain sensors are presented. In Section 4, the applications of printed strain sensors in healthcare, sports performance monitoring, and human-machine interfaces are reviewed. In the last section (Section 5), challenges and outlooks in this emerging field are discussed. Interested readers are referred to recent reviews on strain sensors for aspects that are not covered in this review. [8,9,29] Strain Sensing MechanismsVarious strain sensing mechanisms have been adopted for flexible and stretchable strain sensors, including capacitive, resistive, piezoelectric, triboelectric, and optical. [8,9,30,31] For printed skinattachable strain sensors, capacitive, resistive, and piezoelectric mechanisms are mostly used. [8,9] Materials, internal micro-/ nanostructures, and manufacturing methods are important factors that can influence the response of strain sensors. Conventional thin-foil strain gauges are typically based on geometrical changes, material piezoresistive effects, or piezoelectric effects. [8] With the development of highly stretchable strain sensors, new sensing mechanisms are proposed, including crack propagation, disconnection, and tunneling effect. [7][8][9]17,32] These sensing mechanisms are mainly based on resistance changes caused by strain-induced changes in micro/nanostructures. In this section, we will review these three sensing mechanisms for printed strain sensors (Figure 2). Capacitive EffectStrain sensors based on capacitive effects are basically deformable capacitors. Under tensile strain, the length in the strain direction increases while the cross-sectional area decreases due to the Poisson effect. The geometrical deformation under strain leads to the change in the capacitance. The initial capacitance of capacitive strain sensors is given by Figure 1. Overview of printed strain sensors for on-skin electronics. Clockwise from the top right image: Reproduced with permission.