  Chameleons are light scalar fields with remarkable properties. Through the
interplay of self-interactions and coupling to matter, chameleon particles have
a mass that depends on the ambient matter density. The manifestation of the
fifth force mediated by chameleons therefore depends sensitively on their
environment, which makes for a rich phenomenology. In this article, we review
two recent results on chameleon phenomenology. The first result a pair of no-go
theorems limiting the cosmological impact of chameleons and their
generalizations: i) the range of the chameleon force at cosmological density
today can be at most ~Mpc; ii) the conformal factor relating Einstein- and
Jordan-frame scale factors is essentially constant over the last Hubble time.
These theorems imply that chameleons have negligible effect on the linear
growth of structure, and cannot account for the observed cosmic acceleration
except as some form of dark energy. The second result pertains to the quantum
stability of chameleon theories. We show how requiring that quantum corrections
be small, so as to allow reliable predictions of fifth forces, leads to an
upper bound of m < 0.0073 (\rho/ 10 g cm^{-3})^{1/3} eV for gravitational
strength coupling, whereas fifth force experiments place a lower bound of
m>0.0042 eV. An improvement of less than a factor of two in the range of fifth
force experiments could test all classical chameleon field theories whose
quantum corrections are well-controlled and couple to matter with nearly
gravitational strength regardless of the specific form of the chameleon
potential.
