Wearable and implantable bio-integrated electronics have started to gain momentum because of their essential role in improving the quality of life for various patients and healthy individuals. However, their continuous operation is often limited by traditional battery technologies with a limited lifespan, creating a significant challenge for their development. Thus, it is highly desirable to harvest biomechanical energies from human motion for self-powered bio-integrated functional devices. Piezoelectric energy harvesters are ideal candidates to achieve this goal by converting biomechanical energy to electric energy. Because of their applications on soft and highly deformable tissues of the human body, these devices also need to be mechanically flexible and stretchable, thus posing a significant challenge. Effective methods to address the challenge include the exploration of new stretchable piezoelectric materials (e.g., hybrid composite material) and stretchable structures (e.g., buckled shapes, serpentine mesh layouts, kirigami designs, among others). This review presents an overview of the recent developments in new intrinsically stretchable piezoelectric materials and rigid inorganic piezoelectric materials with novel stretchable structures for flexible and stretchable piezoelectric sensors and energy harvesters. Following the discussion of theoretical modeling of the piezoelectric materials to convert mechanical deformations into electrical signals, the representative applications of stretchable piezoelectric materials and structures in wearable and implantable devices are briefly summarized. The present limitations and future research directions of flexible and stretchable piezoelectric devices are then discussed.