ABSTRACT The author examines Leonardo da Vinci's lifelong interest in the depiction of blurred, colored shadows from the point of view of painting technique and optics. She analyzes Leonardo's refinement of the oil technique to capture the instability of colored shadows from the early Annunciation of 1472–1473 and examines the artist's theoretical writings on shadows from the 1490s onward. The author shows how Leonardo analyzed the blurred edges of colored shadows with the geometric rigor that earlier authors afforded only to the clear-cut edges of astronomical shadows. She argues that the very kind of shadows that captured Leonardo's attention indicates his underlying pictorial concerns, despite the fact that his instructions often seemed directed toward teaching a way of seeing rather than a way of painting.