The magnetic properties of conventional and high-temperature superconductors have been investigated using several neutron scattering techniques. Elastic-neutron diffraction measurements have been performed upon a single crystal of PrBa2Cu3O7-δ to explore the nature of the magnetic order. At Tɴ1, which is above room temperature for all 8, the Cu spins in the CuO2 planes order in a simple antiferromagnetic arrangement with the spins lying within the a-b plane. At a lower temperature, at TN2, the chainer Cu spins also order antiferromagnetically in the a-b plane forcing an overall non- collinear configuration of Cu spins. TN2 was found to be strongly dependent on the oxygen concentration of the crystal. No long-range magnetic ordering of the Pr spins was observed down to 1.9K, although strong two-dimensional short-range correlations of the Pr spins were observed at temperatures up to 20K, with the Pr spins aligning antiferromagnetically along the c direction. The failure of the onset of long-range magnetic order is attributed to frustration in the crystal caused by impurities and substitutional defects. These strong two-dimensional short-range correlations, at uncharacteristically high temperatures, provide possible evidence of the hybridisation of the Pr 4f electrons with the conduction electrons of the CuO2 planes. The magnetisation density induced by an applied field in PrBa2Cu3O7-δ has been measured by polarised-neutron diffraction. The magnetisation density distribution at the Pr site is found to differ strongly from a spherical free-ion distribution with the magnetisation density extending towards the Cu sites in the CuO2 planes. This distortion from a free-ion density distribution again supports the hypothesis of the hybridisation of the Pr 4f electrons with the conduction electrons of the CuO2 planes. Polarised-neutron reflectrometry has been used to study the evolution of the magnetic induction profile at the surface of a superconducting Pb film with increasing applied field. In the bulk superconducting state the spin- dependent reflectivity profiles are consistent with a pure exponential decay of the magnetic induction with a penetration depth of (390 ± 10) A. In the surface superconducting region the reflectivity profiles are accurately described over the whole range of applied fields by the Ginsburg-Landau theory if the Ginsburg-Landau parameter, K, is allowed to vary.