Abstract: Long-wave infrared (LWIR, 8–12 μm) thermal imaging technology holds great promise for applications in security surveillance, medical diagnostics, intelligent transportation, and beyond. Reducing the size and weight of LWIR optical systems are critical to its advancement. Flat lenses based on microstructures, including metasurfaces and multilevel diffractive lenses, offer an intriguing route toward lightweight LWIR optics. This review provides a systematic overview of the working principles, key performance metrics, and differences between diffractive lenses and metalenses. The material properties and microfabrication methods of commonly used LWIR optical materials are summarized. Furthermore, recent progress in micro-fabrication techniques of LWIR flat lenses, including large-area pattern transfer and high-aspect-ratio microstructure etching, is reviewed in detail. Finally, the challenges facing the future development of LWIR flat lenses are discussed.