A laser is an optical device that transforms incoherent input energy (the pump), into coherent outgoing radiation in a specific set of modes of the electromagnetic field, with distinct frequencies. There is a threshold pump energy for the first lasing mode, and above that energy the laser is a non-linear device, and non-linear interactions strongly affect the emission properties of the laser. Surprisingly, the electromagnetic theory of non-linear steady-state multimode lasing remained rather rudimentary until recently. Motivated by the complex laser cavities being developed in modern micro and nano-photonics, we have developed a new formalism, Steady-state Ab initio Laser Theory (SALT), which calculates directly all the steady-state lasing properties without integration of the semiclassical lasing equations in time, and is hence orders of magnitude more efficient, and provides additional analytic insight. The method is based on non-hermitian states of the electromagnetic field, and allows us to treat complicated and unconventional lasers, such as chaotic cavity, and random lasers. Our reformulation of laser theory emphasizes that a laser cavity is a certain kind of scattering system, with a non-unitary amplifying scattering matrix due to the presence of gain. This approach suggested the possibility of constructing a time-reversed or “anti-laser”, which we term a coherent perfect absorber (CPA); a linear device in which the gain medium of the laser is replaced with a loss medium such that the cavity will perfectly absorb the incoming (time-reversed) modes of the corresponding laser at threshold. Recently we have experimentally demonstrated such a device in a simple silicon cavity, which acts as an absorptive interferometer, in which narrow-band absorption can be both increased to ~ 99% and reduced to ~30%. Theoretical extensions of this concept indicate that an appropriately prepared input radiation pattern can be made to penetrate deeply into an opaque elastic scattering medium, where it can be completely captured by a “buried absorber”.