"A numerical study of dynamics, synchronization and applications of semiconductor lasers with external cavities in secure communication systems is presented in this dissertation. The dynamics of an external-cavity semiconductor laser is studied through bifurcation diagrams using the feedback strength, length of the external cavity and the initial phase of the electric field as control parameters. A second cavity is then added, and it is showed that it is a simple mechanism to stabilize chaotic orbits into fix points. Synchronization between a master and a slave external-cavity semiconductor laser is studied for different conditions. It is first studied in monostable regions where these lasers are operating in a CW, periodic or chaotic regime. Then the synchronization is studied in regions of multistability. The applications of external-cavity semiconductor lasers in secure communications is then studied when these lasers behave in a chaotic way. A one-channel communication system (all-optical) is compared with a two channel communication system (all optical and optoelectronic) using different message transmission techniques. The outline of this dissertation is as follows: In chapter 1 a brief introduction of lasers, external-cavity semiconductor lasers, synchronization and communication systems is given. In chapter 2 the dynamics of an external-cavity semiconductor laser is analyzed according to different control parameters. Chapter 3 is dedicated to the study of optical injection from a master laser (ML) to a slave laser (SL). Different scenarios are looked according to the dynamics of the ML or the SL; not only when they have chaotic behavior, but also when they have fix point or periodic regimes are studied. We study the case where lasers are operating in monostable and multistable regions. On chapter 4 applications of synchronization of chaotic lasers in secure optical communications are studied. Different scenarios, like one channel and two channels using pure optical elements, and a two-channel optoelectronic system are studied. Finally, global conclusions of the results are given."