We present the results of inelastic neutron scattering experiments on quantum spin systems KCuCl 3 and SrCu 2 (BO 3 ) 2 . Both compounds show non-magnetic singlet ground state behaviour at low temperature. KCuCl 3 turns out to be a weakly coupled dimer system without any strong frustrating interdimer interactions. SrCu 2 (BO 3 ) 2 , on the other hand, should be regarded as a strongly coupled dimer system with a strong frustration originating from the orthogonal arrangement of the neighbouring dimers. The characteristics of the spin excitations in these two dimer systems are compared and discussed.