The perception of objects is a well-developed field, but the perception of materials has been studied rather little. This is surprising given how important materials are for humans, and how important they must become for intelligent robots. We may learn something by looking at other fields in which material appearance is recognized as important. Classical artists were highly skilled at generating convincing materials. The simulation of material appearance is a topic of great importance in 3D computer graphics. Some fields, such as mineralogy, use the concept of a 'habit' which is a combination of shape and texture, and which may be used for characterizing certain objects or materials. We have recently taken steps toward material recognition by machines, using techniques derived from the domain of texture analysis.