  A pseudo-Nambu-Goldstone boson, with a potential of the form $ V(\phi) =
\Lambda^4 [1 + \cos(\phi/f)]$, can naturally give rise to an epoch of inflation
in the early universe (Freese, Frieman, and Olinto 1990). The potential is
naturally flat (as required by microwave background limits on the amplitude of
density fluctuations), without any fine-tuning. Successful inflation can be
achieved if $f \sim m_{pl}$ and $\Lambda \sim m_{GUT}$. Such mass scales arise
in particle physics models with a large gauge group that becomes strongly
interacting at a scale $\sim \Lambda$, {\it e.g.,} as can happen in the hidden
sector of superstring theories. The density fluctuation spectrum can be
non-scale-invariant, with more power on large length scales (Adams, Bond,
Freese, Frieman, and Olinto 1993). This enhanced power on large scales may be
useful to explain the otherwise puzzling large-scale clustering of galaxies and
clusters and their flows. Natural inflation differs from other models with
extra large-scale power in that the contribution of the tensor modes to
microwave background fluctuations is negligible; this difference should serve
as a testable feature of the model.
