During the last decades, advancement in the development of different small-sized, portable, low-power, and remote systems has attracted the attention of researchers to employ nonconventional power sources. Piezoelectric and triboelectric nanogenerators are two newly developed technologies for efficient harvesting of environmental, mechanical energy for self-powered systems. This book chapter reviews recent progress in energy harvesting devices based on piezoelectric and triboelectric mechanisms. Different materials and fabrication methods in developing piezoelectric, as well as triboelectric energy harvesting devices, are discussed in this review and fabrication of flexible and hybrid devices for different energy harvesting applications are thoroughly reviewed. In addition, recent research directions being studied, the developing parameters to improve harvester's performance and advance growth in attaining high functionality and durable energy transformation are offered.