The technological progress in optical systems demands for ever higher flexibility, efficiencies and compactification. In the course of the development of such photonic systems with ever increasing performances non-linear effects in thin films are gaining in interest. The present contribution is addressed to third order non-linearities in thin films and their applications as an alternative for crystals generally used for third harmonic generation. Contemporary third harmonic sources are predominantly based on a two-step concept involving the generation of the second harmonic in the first crystal and a subsequent sum frequency process to the third harmonic employing the second harmonic and the fundamental in a second crystal. Consequently, third harmonic sources are relatively complex and expensive. As a possible alternative, a direct conversion by a third order process in bulk media can be considered. However, with respect to the dispersion in the materials a sufficient phase matching is rather difficult to achieve, and typical conversion lengths are in the range of a few microns. Applying thin film stacks, this physical limitation can be compensated by a well-designed phase matching in the stack, where the active layers produces the third harmonic and the inactive layers control the phases. In addition the field strength can be locally inflated which allows gaining higher conversion efficiencies in the layer stack. The generation of third order harmonics is a highly non-linear process and the efficiency depends on intensity of the input beam, which is limited by the damage threshold of the materials. Laser induced damage and fatigue effects in thin film components are still topics of major importance. Consequently, research in coating materials and system design properties to improve the power handling capability of optics is one of the high priority topics in thin film technology nearly since the development of the first laser sources. In the present study, different design concepts for coatings systems optimized to third harmonic generation are presented. The production of the complex dielectric stacks, the stability of the conversion process, and the conversion efficiency are considered. The designs impose high demands on both, the choice of materials as well as the precision of the deposition process. The theoretical approaches are discussed before the background of the power handling capability and fatigue effects of the involved coating materials, and the realized structures are qualified in experimental studies. In conclusion, the potential of nonlinear thin film components as a compact, cost efficient and stable solution for future applications is illustrated.