The formation of magnetic order in solids is a complex and subtle issue. There are a wide range of different types of magnetisation, all of which may be favoured under different circumstances. In this thesis we consider a novel combination of ideas where the formation of spatially modulated magnetisation is linked to a metamagnetic transition. In this we are inspired by a general principle of modulated phases intervening as intermediate states in phase transitions. In particular we draw analogies with the Fulde-Ferrell-Larkin-Ovchinnikov state of spatially modulated superconductivity. We study a mean-field theory for itinerant magnetism where the crystal lattice drives the formation of a rich phase diagram. A peak in the electronic density of states due to a van Hove singularity creates ferromagnetism and a metamagnetic transition. Furthermore we find that a modulated magnetic phase - a spin-spiral, becomes favoured along the metamagnetic transition line. The appearance of this phase causes the metamagnetic transition to bifurcate to enclose the modulated region. The topology of this reconstructed phase diagram shows remarkable similarity to that observed in experiments on Sr₃Ru₂O₇. This material shows a metamagnetic transition which can be tuned by field angle towards zero temperature. Before this point is reached a new phase with high and anisotropic resistivity appears. We believe that this anomalous phase can be explained by the formation of a phase of modulated magnetisation caused by a peak in the electronic density of states. This mechanism may also apply in a range of other materials as it is driven by rather generic features of the bandstructure.