Regardless of their geometrical structure, semiconductor lasers are sensible to internal and external perturbations. When part of the emitted light is reflected back into the laser cavity, the laser is said to be subject to optical feedback. A semiconductor laser may be injected by another laser, in which case it is said to be subject to external optical injection. Instead of the steady-state dynamics typical for a free-running laser, the output characteristic of a semiconductor laser subject to either one of the external perturbations can exhibit a wide range of nonlinear instabilities. Nonlinear dynamics in semiconductor lasers may also be caused internally by current modulation. Another perturbation may arise from bias pumped currents. For example, when a specific control parameter associated with a given perturbation is adequately swept, the laser may undergo periodic dynamics, self-pulsating or even more complex chaotic dynamics. The main reason for the high interest in semiconductor lasers which are also known as class-B lasers is a deeper understanding of their coherent dynamical behavior. This understanding contributes to their optimal use.