Adults are skilled at perceiving subjective contours in regions without any local image information (e.g., [Ginsburg, 1975] and [Kanizsa, 1976]). Here we examined the development of this skill and the effect thereon of the support ratio (i.e., the ratio of the physically specified contours to the total contour length). Children (6-, 9-, and 12-year-olds) and adults discriminated between fat and skinny shapes formed by subjective or luminance-defined contours. By 9 years of age, children were as sensitive as adults to small differences in luminance-defined contours, but not until 12 years of age were children as sensitive as adults in performing the same task with subjective contours. Remarkably, 6-year-olds' sensitivity to subjective contours was independent of the support ratio, unlike that of older children and adults. The results suggest that, during middle childhood, the interpolation of subjective contours becomes tied to the support ratio, so that contours that are more likely to reflect the contours of real objects (i.e., highly supported contours) are more easily interpolated. (Contains 4 figures.)