We describe a lighting design system driven by sketching on 3D virtual models. Conventional lighting design tools simulate the lighting effects of design decisions such as window locations, surface treatments, and fixture placement. Light Pen takes the inverse approach by allowing the designer to indicate desired illumination on a 3D model. This serves as input to a knowledge-based lighting design system, which recommends what lights to use and where to place them, based on the designer's expressed intentions and on the geometry of the space.