A brief review is presented on the ac losses of superconductors in the low-frequency, high-frequency, and weak-field regions. Special emphasis is given to the hysteresis loss which is the main part of the low frequency losses. Existing data of the low-frequency losses are collected to be shown in a single figure and it is shown that most of these can be explained by the theories developed up to now. Descriptions are also given on flux-flow losses which become dominant in the high-frequency region. It is noted that losses exist even in the field lower than Hc1 in type-II materials (or Hc in type-I materials), and possible mechanisms for these losses are suggested. Lastly some remarks are mentioned on the applications of superconductors to the ac uses.