  A slow-rolling scalar field (Quintessence) has been proposed as the origin of
accelerating universe at present. We discuss some features of quintessence,
particularly those related with the {\it flatness} of its potential. We
distinguish two types of quintessences, a metrical quintessence which couples
to the particle physics sector {\it only} through the mixing with the spacetime
metric and a non-metrical one without such property. It is stressed that these
two types of quintessences have quite different features in regard to the fine
tuning of parameters required for the flat potential, as well as different
implications for the quintessence-mediated long range force. The fine tuning
argument strongly suggests that non-metrical quintessence should correspond to
the Goldstone boson of an almost exact nonlinear global symmetry whose explicit
breaking by quantum gravity effects is highly suppressed. It is briefly
discussed whether the radion field in higher dimensional theories can be a
candidate for metrical quintessence. We finally discuss the possibility that
string/$M$ theory axion corresponds to a Goldstone-type quintessence, and argue
that a certain combination of the heterotic $M$ theory or Type I string theory
axions can be a good candidate for quintessence if some conditions on the
moduli dynamics are satisfied.
