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Self-powered devices and micro-sensors are in high demand for intelligent electronics and flexible wearables for applications in medical healthcare and human–computer interactive robotics. Flexible, stretchable, wearable and breathable high-sensitivity sensors that monitor signals from subtle changes in the environment provide solutions for personalized medical healthcare. In this article, we review the fundamental mechanisms, theoretical research, sensor fabrication methodologies, and applications of flexible piezoelectric materials. We focus on the flexibility and stretchability of inorganic, polymer, and bio-piezoelectric materials, explain their properties for physiological signal monitoring, motion detection, and force sensing. We provide an overview of the latest progress in piezoelectronics and piezoelectric photonics, the structures of devices, and self-powering technologies. We compared 10 types of polymers and composites with human skin in terms of elastic modulus and found that PLLA/PDMS, electro-spun PVDF, and ZnO/PVDF possess higher elastic modulus (50, 500, and 27900 MPa respectively) than human skin (18.8 MPa), thus being more suitable for wearable devices. In terms of electrical outputs of piezoelectric materials, polarized cast film has stronger piezoelectric property with output of 14–45.6 V than electro-spun filaments material (output 0.4–9 V). Future research and development should focus on increasing electrical performance while maintaining the required flexibility and durability.