Summary form only given. Hole-induced ferromagnetism in transition metal doped III-V compounds offers integration of ferromagnetism with the existing nonmagnetic III-V heterostructures. These structures allow us to explore spin-dependent phenomena in semiconductor heterostructures, which may lead us to a new form of electronics, semiconductor spintronics, where both the spin and charge degrees of freedom play critical roles (H. Ohno, Science, vol. 281, p. 951, 1998; and J. Mag. Mag. Materials, vol. 200, p. 110, 1999). Here, the author reviews recent developments in the field of III-V ferromagnetism and spin-dependent phenomena in its heterostructures, with an emphasis on electric field manipulation of ferromagnetism in FET structures. A mean-field theory (T. Dietl et al., Science, vol. 287, p. 1019, 2000; T. Dietl et al., Phys. Rev. B vol. 63, pp. 195-205, 2001) based on exchange between carrier spin and Mn spin capable of explaining the ferromagnetic transition temperatures and strain-dependent easy axis indicates that the properties of hole-induced ferromagnetism in magnetic III-V's depend critically on the hole concentration. By the use of insulating-gate field-effect transistor structures, we have demonstrated electrical switching of the ferromagnetic phase transition (H. Ohno et al., Nature, vol. 408, p. 944, 2000). We are thus beginning to learn how to control and utilize the spin degree of freedom in semiconductors. At present this is a low temperature effect. Routes to room temperature ferromagnetism are also discussed.