Many common materials, including fruit, wax and human skin, are somewhat translucent. What makes an object look translucent or opaque? Here we use a recently developed computer graphics model of subsurface light transport [Jensen, et al., 2001] to study the factors that determine perceived translucency. We discuss how physical factors, such as light-source direction can alter the apparent translucency of an object, finding that objects are perceived to be more translucent when illuminated from behind than in front. We also study the role of a range of image cues, including colour, contrast and blur, in the perception of translucency. Although we learn a lot about images of translucent materials, we find that many simple candidate sources of information fail to predict how translucent an object looks. We suggest that the visual system does not rely solely on these simple image statistics to estimate translucency: the relevant stimulus information remains to be discovered.