This review focuses on half-Heusler MNiSn alloys (M = Ti, Zr, Hf), which are of particular interest for creation of thin-film thermoelectric generators (TEGs) operating in the low- and medium-temperature ranges (300 – 800 K). The paper examines current energy challenges requiring the development of compact and efficient TEGs, as well as the reasons for the increased attention paid to half-Heusler alloys compared to traditional Bi-, Te-based materials. The crystal structure, phase stability, electronic and phononic properties of the alloys, synthesis methods, and alloying strategies are discussed in detail. The results of recent studies on thin-film MNiSn are analyzed, and their potential for flexible TEG architecture, opening new avenues for highly efficient miniature energy devices, is assessed.