  We present a general study on exclusive semileptonic decays of heavy (B, D,
B_s) to light (pi, rho, K, K^*) mesons in the framework of effective field
theory of heavy quark. Transition matrix elements of these decays can be
systematically characterized by a set of wave functions which are independent
of the heavy quark mass except for the implicit scale dependence. Form factors
for all these decays are calculated consistently within the effective theory
framework using the light cone sum rule method at the leading order of 1/m_Q
expansion. The branching ratios of these decays are evaluated, and the heavy
and light flavor symmetry breaking effects are investigated. We also give
comparison of our results and the predictions from other approaches, among
which are the relations proposed recently in the framework of large energy
effective theory.
