Combination of CDF and D0 Results on the Top-Quark Mass

The results on the measurements of the top-quark mass, based on the data collected by the Tevatron experiments CDF and D{\O}at Fermilab during Run I from 1992 to 1996, and Run II since 2001 are summarized. The combination of the published Run I and preliminary Run II results, taking correlated uncertainties properly into account, is presented. The resulting preliminary world average for the mass of the top quark is: $\MT=174.3 \pm 3.4 \GeVc2$, where the total error consists of a statistical part of $2.0 \GeVc2$ and a systematic part of $2.8 \GeVc2$.