Alloys between non-magnetic III-V semiconductors and Mn have been grown by molecular beam epitaxy and shown to exhibit ferromagnetism at reduced temperatures. These alloys, (Ga,Mn)As, (In,Mn)As, and (Ga,Mn)Sb are quasi-lattice matched to their host semiconductors and thus offer new and unique opportunities to combine ferromagnetism and high quality III-V heterostructures being widely used in frontiers of semiconductor physics and also in commercially available devices. This talk covers the following topics: (1) Preparation and properties of ferromagnetic semiconductors, particularly (Ga,Mn)As, where transition temperature can be as high as 110 K for 5% Mn concentration.@footnote 1@ (2) The origin of carrier-induced ferromagnetism based on a mean field theory using kp approximation.@footnote 2@ (3) Ferromagnet/non-magnet tri-layer semiconductor structures exhibiting inter-layer magnetic coupling and spin-dependent scattering.@footnote 3@ (4) Resonant tunneling structures with ferromagnetic emitters.@footnote 4@ (5) Spin-injection experiments using ferromagnetic semiconductor heterostructures.@footnote 5@ (6) Spin relaxation in nonmagnetic (110) GaAs quantum wells, where prolonged spin relaxation times are observed,@footnote 6@ which can be over 10 ns from room temperature down to 5 K when modulation doped. @FootnoteText@ @footnote 1@ H. Ohno, Science, 281, 951 (1998), J. Mag. Mag. Materials, 200, 110 (1999). @footnote 2@ T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science, 287, 1019 (2000). @footnote 3@ N. Akiba, D. Chiba, K. Nakata, F. Matsukura, Y. Ohno, and H. Ohno, J. Appl. Phys., 87, 6436 (2000). @footnote 4@ H. Ohno, N. Akiba, F. Matsukura, A. Shen, K. Ohtani, and Y. Ohno, App. Phys. Lett., 73, 363 (1998). @footnote 5@ Y. Ohno, D. K. Young, B. Beschoten, F. Matsukura, H. Ohno, and D. D. Awschalom, Nature, 402, 790 (1999). @footnote 6@ Y. Ohno, R. Terauchi, T. Adachi, F. Matsukura, and H. Ohno, Phys. Rev. Lett., 83, 4196 (1999).