Half-Heusler alloys are among the most promising thermoelectric materials. In the present review, thermoelectric properties (at 300 K and 800 K) of more than 1100 compositions from more than 220 publications between 1998 and 2023 were collected and evaluated. The dependence of the peak figure of merit, ZTmax, of p- and n-type half-Heusler alloys on the publishing year and the peak temperature is displayed in several figures. Furthermore, plots of ZT vs. the electrical resistivity, the Seebeck coefficient and the thermal conductivity at 300 K and 800 K are shown and discussed. Especially thermal conductivity vs. power factor leads to a good overview of ZT. For both p- and n-type individually separated into systems, ZTs and peak ZTs in dependence on the composition are displayed and discussed. This overview can help to find the ideal half-Heusler alloy for practical use.