"For nearly a century, astronomers have examined galaxies. We have observed millions of them, sometimes in great detail, at wavelengths ranging from radio to x ray. Most observations have been geared toward measuring basic galactic properties or classifying galaxies based on how they appear on photographs; only recently have we begun to explore their underlying physics. Important unresolved issues include the persistence over long periods of time of structures such as spirals in the Milky Way, the nature of the nonluminous dark matter that dominates the mass of galaxies, and the reasons galaxies cluster together. Another major question relates to all those issues and more, and still awaits a complete answer: How do galaxies form and evolve? That question is the focus of this Quick Study."