Some varieties of shadow illusions: split shadows, occluded shadows, stolen shadows, and shadows of shadows.

Shadows are visual objects and as such are subject to preference rules for segmenting the visual scene (such as Gestalt laws). These rules govern shadows along two dimensions: their general unity and individuation (eg they describe a certain area as a unitary shadow as opposed to a set of two distinct shadows), and their being the shadow of a given object, as opposed to being the shadow of any other object in the scene (the shadow correspondence problem Mamassian, 2004 Perception 33 1279-1290). I describe a few phenomena that indicate the interplay of preferences in determining the final visual aspect of scenes in which shadows are present.