properties were inferred from magnetization dependent Hall measurements of the channel. Using the gate action to either deplete or accumulate holes in the channel it was possible to change the characteristic of the Hall response to be either that of a paramagnet or of a ferromagnet. When the temperature of the sample was close to its TC it was possible to turn the ferromagnetism on or off by applying a gate voltage which could change the TC by ±1°K. A similar transistor device was used to provide further examples of gateable ferromagnetism. In this experiment the electric field was used to modify the coercive field at which magnetization reversal occurs. As a result of the dependence of the magnetic hysteresis on the gate bias the electric field could be used to assist magnetization reversal or even demagnetize the ferromagnetic material. The combining of magnetic and electronic functionality demonstrated by this experiment is one of the goals of spintronics and may be expected to have a great technological impact. Another important spintronic functionality that has been demonstrated in magnetic semiconductors is that of spin injection. This is where the high spin polarization inherent to these magnetic materials is used to transfer spin polarized carriers into a non-magnetic material. In this example, a fully epitaxial heterostructure was used where spin polarized holes were injected from a layer to an (In,Ga)As quantum well where they combine with unpolarized electrons from an n-type substrate. A polarization of 8% was measured in the resulting electroluminescence. This is again of potential technological interest as it shows the possibility that the spin states in non-magnetic semiconductors can be manipulated without the application of a magnetic field. offers an excellent material to study domain wall mechanics because the domains can have a size of the order of 100 μm. Several studies have been done in which lithographically defined lateral constrictions or other pinning points are used to manipulate domain walls. These experiments are crucial to understanding domain wall nucleation and propagation which would be necessary for the creation of complex logic circuits based on domain wall mechanics. Many properties of domain walls are still not fully understood and one particularly outstanding issue is of the magnitude and size of the resistance associated with current passing through domain walls. Both positive and negative values of domain wall resistance have been reported, leaving this an open area for future research.