Most solids begin to melt at temperatures well below the bulk transition, by forming surface layers of liquid a few molecules thick. Although it is difficult to detect by direct observation, the phenomenon known as surface melting is believed to play important roles in many technical and natural processes, including the processing of powdered metals and ceramics, crystal growth, and the compaction and friction of snow and ice. The theory of surface melting describes how the effect depends on the fundamental properties of the bulk material and its interfaces. Detailed observations of surface melting have been obtained within the past few years, confirming the predictions of theory and raising new questions.