Advanced ultrasound energy transfer technologies using metamaterial structures are being explored for efficient wireless energy transfer (WET) through ultrasound-driven generators. This review discusses the fundamentals, classification, and design engineering of ultrasonic metamaterials (UMMs) and their applications in ultrasound energy transfer (US-ET) systems. UMMs are engineered materials that manipulate ultrasound waves based on their design, rather than their constituent materials, enabling efficient conversion of ultrasound energy into electrical energy. These materials are particularly useful in enhancing the performance of piezoelectric and triboelectric nanogenerators (PENGs and TENGs) for applications such as self-powered implants, wearable devices, and underwater communication. UMMs can be classified into sound focuser, sound absorber, sound cloak, and sound refractive types, each with specific functions in manipulating ultrasound waves. The design and fabrication of UMMs involve careful consideration of material properties, structural design, and fabrication techniques. Common materials include metals, polymers, composites, and ceramics, with advanced options like piezoelectric materials offering additional functionalities. UMMs can be fabricated using techniques such as microfluidic wet-chemistry, molding, 3D printing, and microfabrication. In energy harvesting applications, UMMs are used to enhance the efficiency of US-ET systems by focusing and scavenging energy from ambient ultrasound sources. The integration of UMMs into US-ET systems can improve the conversion of mechanical energy (sound waves) to electricity. For example, piezoelectric receivers convert mechanical vibrations of ultrasonic waves into electrical energy using the piezoelectric effect, while triboelectric nanogenerators (TENGs) use mechanical vibrations to induce triboelectric charging and generate electrical energy. The review highlights the potential of UMMs in improving the efficiency and power output of US-ET systems, enabling the development of self-powered medical implants and wearable devices. The use of UMMs in US-ET systems is still an emerging field, with ongoing research focused on developing novel designs, fabrication techniques, and applications for UMMs. The integration of UMMs into US-ET systems is crucial for achieving high-efficiency energy transfer and enhancing the capabilities of ultrasound-based energy harvesting technologies.Advanced ultrasound energy transfer technologies using metamaterial structures are being explored for efficient wireless energy transfer (WET) through ultrasound-driven generators. This review discusses the fundamentals, classification, and design engineering of ultrasonic metamaterials (UMMs) and their applications in ultrasound energy transfer (US-ET) systems. UMMs are engineered materials that manipulate ultrasound waves based on their design, rather than their constituent materials, enabling efficient conversion of ultrasound energy into electrical energy. These materials are particularly useful in enhancing the performance of piezoelectric and triboelectric nanogenerators (PENGs and TENGs) for applications such as self-powered implants, wearable devices, and underwater communication. UMMs can be classified into sound focuser, sound absorber, sound cloak, and sound refractive types, each with specific functions in manipulating ultrasound waves. The design and fabrication of UMMs involve careful consideration of material properties, structural design, and fabrication techniques. Common materials include metals, polymers, composites, and ceramics, with advanced options like piezoelectric materials offering additional functionalities. UMMs can be fabricated using techniques such as microfluidic wet-chemistry, molding, 3D printing, and microfabrication. In energy harvesting applications, UMMs are used to enhance the efficiency of US-ET systems by focusing and scavenging energy from ambient ultrasound sources. The integration of UMMs into US-ET systems can improve the conversion of mechanical energy (sound waves) to electricity. For example, piezoelectric receivers convert mechanical vibrations of ultrasonic waves into electrical energy using the piezoelectric effect, while triboelectric nanogenerators (TENGs) use mechanical vibrations to induce triboelectric charging and generate electrical energy. The review highlights the potential of UMMs in improving the efficiency and power output of US-ET systems, enabling the development of self-powered medical implants and wearable devices. The use of UMMs in US-ET systems is still an emerging field, with ongoing research focused on developing novel designs, fabrication techniques, and applications for UMMs. The integration of UMMs into US-ET systems is crucial for achieving high-efficiency energy transfer and enhancing the capabilities of ultrasound-based energy harvesting technologies.