Hafnia (HfO2)-zirconia (ZrO2) solid solution films show giant positive (Delta T = 13.4 K) and negative (Delta T = -10.8 K) electrocaloric effects that can be simply controlled by tuning the Hf/Zr ratio. It is expected that the combination of the electrocaloric effects with opposite signs in this lead-free, simple, binary oxide can significantly improve the efficiency of electrocaloric cooling.