Piezoelectric energy harvesting (PEH) is recognized as a promising solution for converting vibration energy into usable electricity, because it has high energy density and can be easily miniaturized for use in wireless sensor nodes. Metamaterial-based energy harvesting (MBEH) has been recently proposed as a breakthrough technology to drastically enhance the output electric power generated by a piezoelectric device. The metamaterial refers to an artificial structure engineered to possess exotic properties such as a phononic bandgap. The phononic bandgap is defined as a certain range of frequencies within which mechanical waves cannot be transmitted through phononic crystals (PnC). The phononic bandgap can be used to focus and/or localize mechanical waves on the piezoelectric device for highly-dense energy harvesting. This talk mainly discusses about how the phononic bandgap contributes to the enhancement of energy harvesting performance. Two innovative design concepts for the MBEH using the phononic bandgap will be presented as: (1) a quarter-wave stack (QWS) parabolic mirror for energy focusing and (2) a defect-mode PnC for energy localization.