The chapter discusses various types of metal-insulator transitions in both non-crystalline and crystalline systems. In non-crystalline systems, a second-order "Anderson" transition can occur without considering electron-electron interactions. In crystalline systems, band-crossing transitions and "Mott" transitions are described, both of which involve a discontinuity in the number of carriers when electron-electron interactions are included. The difficulty of directly observing this discontinuity is highlighted, and sufficient disorder in the system can remove it. The behavior of materials such as fluid caesium, metal-ammonia, and expanded fluid mercury is contrasted with Si:P. Several crystalline compounds of transition metals, including Tl₂O₃, VO₂, V₂O₃, NiS₂, and NiS, are discussed, each showing a different mechanism for the transition. A new suggestion is made for Tl₂O₃, and a brief mention is given to the Verwey transitions in Fe₀.₄. The chapter also covers the behavior of electrons in non-periodic fields, the Anderson model, and the theoretical framework for understanding metal-insulator transitions in both non-crystalline and crystalline systems.The chapter discusses various types of metal-insulator transitions in both non-crystalline and crystalline systems. In non-crystalline systems, a second-order "Anderson" transition can occur without considering electron-electron interactions. In crystalline systems, band-crossing transitions and "Mott" transitions are described, both of which involve a discontinuity in the number of carriers when electron-electron interactions are included. The difficulty of directly observing this discontinuity is highlighted, and sufficient disorder in the system can remove it. The behavior of materials such as fluid caesium, metal-ammonia, and expanded fluid mercury is contrasted with Si:P. Several crystalline compounds of transition metals, including Tl₂O₃, VO₂, V₂O₃, NiS₂, and NiS, are discussed, each showing a different mechanism for the transition. A new suggestion is made for Tl₂O₃, and a brief mention is given to the Verwey transitions in Fe₀.₄. The chapter also covers the behavior of electrons in non-periodic fields, the Anderson model, and the theoretical framework for understanding metal-insulator transitions in both non-crystalline and crystalline systems.