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Abstract.A fundamental problem in computational geometry is determining whether a point is inside a B-rep solid. Methods currently used for such point classification are unreliable or inefficient or both. A new approach is illustrated by showing how a simple method for loops of planar curves represented by B-splines can be extended from two dimensions to three. The plan in two dimensions is to construct a polygon so that the point will be inside the loop if and only if it is inside the polygon. Once such a polygon is found, it is easy to compute its winding number with respect to the point. In three dimensions, an analogous (although more complicated) method is robust and efficient.