We present results for the semi-leptonic and leptonic decays of B mesons. These non-perturbative matrix elements are important for constraining the CKM matrix. Results are presented for the pseudoscalar and vector decay constants, as well as flavour breaking ratios and heavy quark symmetry relations. We consider the chiral and momentum dependence of the semi-leptonic form factors of the decay B → πιν and the soft pion relation (SPR) on the lattice. These calculations were performed in the quenched approximation at two values of the coupling with non-perturbatively O( a) improved action and currents.