In a letter written in 1867, James Clerk Maxwell described ahypothetical creature: a “neat-fingered being” capable ofseparating fast molecules from slow ones. Maxwell mused thatsuch a creature would seem to violate the second law ofthermodynamics, which had recently been enunciated byRudolf Clausius and is now a pillar of our understanding of thenatural world. Over the past century and a half, that hypothetical creature – Maxwell’s demon – has wandered throughthe thoughts of eminent scientists, has appeared in researcharticles and popular cultural references, and in recent years hasbeen observed in laboratory experiments. Along the way, themischievous devil has sharpened our understanding of thesecond law of thermodynamics, exposing a deep relationshipbetween physics and information. I will give an overview of thequestions raised and the lessons learned from contemplatingMaxwell’s demon, and I will summarize our current understanding of this topic. This story highlights the importance ofimagination and whimsy in scientific discovery.