Semiconducting half-Heusler compounds with the valence electron count of 18 have been identified as a class of promising high-temperature thermoelectric materials. Recently, nominal 19-electron half-Heusler compounds, traditionally regarded as metals, have gained reacquaintance and popularity due to their unexpected high thermoelectric performance and fascinating defective structure. In this Perspective, we summarize the current progress of 18-electron half-Heusler thermoelectric materials and focus on the discovery and challenge of the cation-deficient 19-electron half-Heusler compounds with the vacancy-related short-range order. Further outlook on the discovery of promising half-Heusler thermoelectrics and the insightful understanding of the defect-tailored thermoelectric properties are offered.