What is time? Any child knows the answer, and yet even the most advanced theoretical physicist is hard put for a fully satisfactory definition. He cannot say for certain when time started, when it will end, or even if it really exists in a philosophical sense. Yet the measurement of time is the basis of all science, for the scientist can study only what changes with time. Astronomers chart the history of the universe in "big time," the passage of billions of years. Physicists and engineers subdivide "little time" into billionths of a second. Biologists have discovered that animals and plants measure time, too - that even the lowliest of single-celled organisms depend on biological clocks to keep themselves synchronized internally and externally. This book explores all of these facets of the meaning of time.