A superconducting magnet for producing a uniform magnetic field, e.g. for NMR imaging, includes a main coil 3 comprising superconducting coils, an active shield 2 comprising superconducting coils which coaxially surround the main coil 3, and a passive shield 1 of magnetic material coaxially surrounding the active shield 2. The magnetic moment of the active shield 2 is smaller in absolute value than and opposite in direction to the magnetic moment of the main coil 3. The arrangement enable an effective compromise L- be made between weight (of the passive shield) and susperse (of the superconducting coils). Passive shield 1 may form part of a cryostat structure for the superconducting coils. <IMAGE>
