We suppose that all of the structure we see in the sky
-- stars, galaxies, groups and clusters, filaments and voids -- has
arisen initially through the action of gravity on "primordial"
fluctuations whose presence we can infer at the epoch of decoupling
from maps of the microwave background radiation. We will try to
understand how such fluctutaions can survive the conditions before
decoupling and how subsequent structure might be formed in a
"top-down" or "bottom-up" way, depending on whether the dark matter is
predominantly "hot" or "cold". Inflation gives a natural origin for
the fluctuations, and a universe dominated by cold dark matter gives a
good, but not perfect, fit to the observations.