  This short review surveys phenomena observed when a magnetic field is applied
to a system of localised spins on a lattice. Its focus is on frustrated magnets
in dimension $d \geq 2$. The interplay of field and entropy is illustrated in
the context of their unusual magnetocaloric properties, where field-tuned
degeneracies assert themselves. Magnetisation plateaux can reveal the physics
of fluctuations, with unusual excitations (such as local modes, extended string
defects or monopoles) involved in plateau termination. Field-tuning lattice
geometry is the final topic, where mechanisms for dimensional reduction and
conversion between different lattice types are discussed.
