Mechanical metamaterials and phononic crystals enable localizing, focusing, and guiding of elastic or acoustic waves in various ways. Here, we describe the physical mechanisms underpinning wave manipulation and then review the most recent energy harvesting methods for converting localized mechanical wave energy to useable electrical energy. Due to the exceptional wave-matter interactions enabled by the man-made structures, energy is collected more efficiently than through conventional methods. Artificially designed mechanical structures are versatile, especially when used in renewable and ecologically-benign energy transformation, and have a wide array of potential applications.