We review the progress on laser cooling of solids. Laser cooling of ion-doped solids and semiconductors is based on the anti-Stokes fluorescence, where the emitted photons have a mean energy higher than that of the absorbed photons. The thermodynamic analysis shows that this cooling process does not violate the second law, and that the achieved efficiency is much lower than the theoretical limit. Laser cooling has experienced rapid progress in rare-earth-ion doped solids in the last decade, with the temperature difference increasing from 0.3to92K. Further improvements can be explored from the perspectives of materials and structures. Also, theories need to be developed, to provide guidance for searching enhanced cooling performance. Theoretical predictions show that semiconductors may be cooled more than ion-doped solids, but no success in bulk cooling has been achieved yet after a few attempts (due to the fluorescence trapping and nonradiative recombination). Possible solutions are discussed, and net cooling is expected to be realized in the near future.

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