This chapter discusses imagination as an organism having an idea that it seeks to examine in its mind or actions. Imagination can occur as mental imagery, or more creatively, in perceiving something as something else (e.g., "seeing as"). Imagination is, thus, dependent upon perceptual processes. Following a brief history of relevant philosophical and psychological ideas, the author critically reviews evidence of imagination (dreams, mental rotation, cognitive maps, planning, insight, experience projection, pretense) in animals and humans, distinguishing simple imagery, imagining that, and imagining how. Claims that imagination (in the form of mental imagery) is necessary for planning and problem solving are problematic, in that imagination (and planning and problem solving) can occur without mental imagery, proposed mental images may require skills organisms do not have, and gaining novel information from mental imagery is notoriously difficult even for adult humans. Evidence and theories about mental imagery and "seeing as" in humans can provide useful insights into animals' imaginations.