The fuzzy band of light in the night sky was speculated to be distant stars by Democritus around 400 BCE; but it was not until 1610, following the invention of the telescope in 1608 by Hans Lipperhey, that this was œnally veriœed by Galileo, who œrst resolved the Milky Way into millions of stars. Since Hubble determined in the early 20th century that the fuzzy nebulae were distant galaxies, astronomers have carefully attempted to visually classify and catalog galaxies into common sequences. The most famous of these, the Hubble sequence, describes galaxies according to the complexity of their appearance-a sequence that is often mistakenly interpreted as an evolutionary sequence because the scheme smoothly stretches from disks lacking a central bulge, through disk (S0) galaxies that have no apparent structure in the disk component, to elliptical galaxies. The subject of this chapter is to give a modest overview of our current understanding of how galaxies form and evolve from a theoretical perspective and to discuss open questions that will be addressed in future work.