  The approximate symmetries of Quantum ChromoDynamics in the infinite heavy
quark ($Q=c,b$) mass limit ($m_Q \to \infty$) and in the chiral limit for the
light quarks ($m_q \to 0,\;q=\,u,\,d,\,s$) can be used together to build up an
effective chiral lagrangian for heavy and light mesons describing strong
interactions among effective meson fields as well as their couplings to
electromagnetic and weak currents, including the relevant symmetry breaking
terms. The effective theory includes heavy ($Q \bar q$) mesons of both negative
and positive parity, light pseudoscalars, as well as light vector mesons. We
summarize the estimates for the parameters entering the effective lagrangian
and discuss in particular some phenomenologically important couplings, such as
$g_{B^* B \pi}$. The hyperfine splitting of heavy mesons is discussed in
detail. The effective lagrangian allows for the possibility to describe
consistently weak couplings of heavy ($B,\, D$) to light ($\pi,\, \rho, \,
K^*,\,$ etc.) mesons. The method has however its own limitations, due to the
requirement that the light meson momenta should be small, and we discuss how
such limitations can be circumvented through reasonable ansatz on the form
factors. Flavour conserving (e. g. $B^* \to B\, \gamma$) and flavour changing
(e. g. $B \to K^* \, \gamma$) radiative decays provide another field of
applications of effective lagrangians; they are discussed together with their
phenomenological implications. Finally we analyze effective lagrangians
describing heavy charmonium- like ($\bar Q Q$) mesons and their strong and
electromagnetic interactions. The role of approximate heavy quark symmetries
for this case and the phenomenological tests of these models are also
discussed.
