This chapter discusses the theory of stationary spectroscopy under four major sections headings, namely, (i) optical transitions, semiconductor Bloch equations, and linear spectra, (ii) plasma-density-dependent spectra, (iii) electro-optical spectra, and (iv) magneto-optical spectra. The electronic states of quantum confined structures between which the optical transitions can take place are given by a mixture of discrete subband quantum numbers and a d-dimensional continuous momentum vector. The optical transitions are governed by the single-particle reduced density matrix. If a strong pump pulse excites an e-h plasma it will relax to quasi-equilibrium thermal distributions both in the conduction and valence bands by carrier-carrier scattering and by scattering with optical phonons in less than a picosecond. Because the lifetime of the e-h carriers is in direct gap semiconductors typically in the order of nanoseconds, there is a long time interval in which the linear spectra of a test beam are determined by a semiconductor with a thermal high-density electron-hole plasma. In semiconductor microstructures the modifications of the optical spectra by a static electric field F0 applied perpendicular to confining potential walls are very different from the Franz-Keldysh effect in bulk semiconductors. Because of the opposite charges, the field pushes the electron and hole toward the opposite potential walls. Hence the overlap between the corresponding particlein-a-box wave functions is drastically modified. The effect of a static magnetic field B perpendicular to the quantum well plane is particularly strong, because the magnetic field forces the carriers in the well layer into quantized cyclotron orbits.