Since their introduction in the early nineties, Magic Lenses have attracted much interest. Especially in the realm of visualization, the elegance of using a virtual interactive lens to provide an alternative visual representation of a selected part of the data is highly valued. In this report, we survey the literature on interactive lenses in the context of visualization. Our survey (1) takes a look at how lenses are defined and what properties characterize them,(2) reviews existing lenses for different types of data and tasks, and (3) illustrates the technologies employed to display lenses and to interact with them. Based on our review, we identify challenges and unsolved problems to be addressed in future research.