What is it that we can never do? Barrow examines what the limitations on human discovery might be and what we may ultimately find to be unknown, impossible, or unlikely. Science is a great success story, but where will it end? And indeed, will it end? During his exploration, Barrow weaves a canvas of surprises by studying the borders of knowledge. We find that the concept of "impossibility" has played a significant role in our thinking. Surrealism, impossible forms, time travel, the paradoxes of logic, and prospects—all stimulate us to think beyond what is. Using simple explanations, he shows us that impossibility is a profound and powerful concept, that any Universe complex enough to contain conscious beings will include limitations on what they can know about their Universe, and that what we cannot know defines reality in the same way that what we can know does.