Extensive studies of the magnetism of metal hydrides have been carried out in the last three decades. Hydrogenation of metals and intermetallic compounds frequently results in a striking alteration of their electronic and magnetic characteristics and leads to some extraordinary new features. In materials which contain a transition metal, the effect of hydrogen on the magnetic properties is twofold: Firstly, from the band structure point view, since hydrogen contributes two states and only one electron, the Fermi level, and hence the transition metal magnetic moment, is affected. Secondly hydrogen absorption results in increased lattice dimensions, which influences the strength of the interaction between magnetic ions. One of the most striking effects of hydrogen absorption is observed in the intermetallic compounds Th7Fe3H30. In this instance, the parent material is a superconductor, but upon hydrogenation it becomes magnetically ordered. The class of compounds based on R6T23 (R=rare earth, T=Mn, Fe) has shown dramatic magnetic structure change upon hydrogenation. For example, on hydrogenation, Y6Mn23 transforms from a ferrimagnet to an antiferromagnet.The influence of hydrogen on the magnetic properties and crystal structure of rare earth-transition metal-based intermetallic compounds with RT2, RT3, R6T23, R2T14B and R6T13X structure type (R=rare earth, T=Fe, Co or Mn and X=the third, fourth and fifth group of elements in the periodic table) is reviewed. [Copyright &y& Elsevier]