  In recent years, a remarkably simple physical picture of the Lyman-alpha
forest has emerged, which allows detailed predictions to be made and turns the
forest into a powerful probe of cosmology. We point out ways in which such a
picture can be tested observationally, and explore three areas in which the
Lyman-alpha forest can yield valuable constraints: the reionization history,
the primordial mass power spectrum and the cosmological constant or its
variants. The possibility of combining with other high redshift observations,
such as the Lyman-break galaxy surveys, to provide consistency checks and
complementary information is also discussed.
