The concept of optical refrigeration (also known as laser cooling of solids) was described shortly after the birth of quantum mechanics. Interestingly, it was first rejected by many as unphysical and in violation of the laws of thermodynamics. The underlying mechanism is simple and based on anti-Stokes fluorescence where incident light from a coherent (low-entropy) source such as a laser is upconverted into high-entropy fluorescence via absorption or removal of vibrational energy (phonons). Some have called this phenomenon “a laser running in reverse." Since its first experimental observation in 1995, optical refrigeration has advanced greatly. Recently, crystals doped with Yb ions have cooled to an absolute temperature of 155K starting from room temperature, with even lower temperatures possible. Optical refrigeration is the only available technique for attaining cryocooling with an entirely solid-state device. In this talk, I will introduce and review the field, including efforts to attain net cooling in semiconductors. Progress has been driven by materials engineering with emphasis on high quantum efficiency and low parasitic background absorption.