People are remarkably accurate (approaching ceiling) at deciding whether faces are male or female, even when cues from hair style, makeup, and facial hair are minimised. Experiments designed to explore the perceptual basis of our ability to categorise the sex of faces are reported. Subjects were considerably less accurate when asked to judge the sex of three-dimensional (3-D) representations of faces obtained by laser-scanning, compared with a condition where photographs were taken with hair concealed and eyes closed. This suggests that cues from features such as eyebrows, and skin texture, play an important role in decision-making. Performance with the laser-scanned heads remained quite high with 3/4-view faces, where the 3-D shape of the face should be easiest to see, suggesting that the 3-D structure of the face is a further source of information contributing to the classification of its sex. Performance at judging the sex from photographs (with hair concealed) was disrupted if the photographs were inverted, which implies that the superficial cues contributing to the decision are not processed in a purely 'local' way. Performance was also disrupted if the faces were shown in photographic negatives, which is consistent with the use of 3-D information, since negation probably operates by disrupting the computation of shape from shading. In 3-D, the 'average' male face differs from the 'average' female face by having a more protuberant nose/brow and more prominent chin/jaw. The effects of manipulating the shapes of the noses and chins of the laser-scanned heads were assessed and significant effects of such manipulations on the apparent masculinity or femininity of the heads were revealed. It appears that our ability to make this most basic of facial categorisations may be multiply determined by a combination of 2-D, 3-D, and textural cues and their interrelationships.