  A relativistic constituent quark model is adopted to give an unified
description of the leptonic and semileptonic decays of pseudoscalar mesons
(\pi, K, D, D_s, B, B_s). The calculated leptonic decay constants and form
factors are found to be in good agreement with available experimental data and
the results of other approaches. Eventually, the model is found to reproduce
the scaling behaviours of spin-flavor symmetry in the heavy-quark limit.
