We report theoretical investigations on ordering phenomena in interacting dimer systems under the influence of an external magnetic field including the　effects of doping with nonmagnetic impurities. Sufficiently strong magnetic fields close the singlet-triplet energy gap which characterizes a large class of low-dimensional spin systems such as weakly interacting af dimers and spin ladders and induce phases exhibiting long range (resp. quasi long range) magnetic order. On the other hand, doping with nonmagnetic impurities converts some dimers into isolated spins which interact weakly via the assembly of intact dimers and order antiferromagnetically.
 　We study the combined effect of doping and external magnetic field using dimer field theory and a mean field approximation and demonstrate that a finite magnetic field may drive the ordered doped system back to the disordered phase if 3D interactions are not too strong [1]. We discuss the resulting quantum phase diagram as well as the influence of doping on the elementary  excitations and on the order parameters magnetization and sublattice magnetization. We conclude that the reentrant behaviour may be experimentally observed if 3D interactions are not too strong.