A solid-state laser use and gain medium that is a solid, rather than a liquid such as dye lasers or a gas such as gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered separately from solid-state lasers. Generally, the active medium of a solid-state laser consists of a glass or crystalline host material to which is added a dopant such as neodymium, chromium, erbium, or other ions. Many of the common dopants are rare earth elements, because the excited states of such ions are not strongly coupled with thermal vibrations of the crystalline lattice (phonons), and the lasing threshold can be reached at relatively low brightness of pump. There are many hundreds of solid-state media in which laser action has been achieved, but relatively few types are in widespread use. Of these, probably the most common type is neodymium doped YAG. Neodymium-doped glass (Nd:glass) and Ytterbium-doped glasses and ceramics are used in solid-state lasers at extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial confinement fusion. Titanium doped sapphire is also widely used for its broad tunability. This book gathers new research in the field.