Coronal holes are brighter than the quiet Sun in microwaves. Microwave emission from the quiet Sun is optically thick thermal bremsstrahlung from the upper chromosphere. Therefore, the optically thick layer in the coronal hole chromosphere must be hotter than the corresponding layer in the quiet chromosphere. We present microwave and SOHO observations in support of this idea. Because of the availability of simultaneous EUV and microwave images it is now possible to obtain more details of this enigmatic phenomenon. In this paper, we highlight the primary properties of the microwave enhancement in coronal holes and point out some related phenomena. Finally, we summarize the possible explanations of the radio enhancement.