The incredible growth of Fiber Bragg Gratings (FBGs) from both the research and the commercial points of view, has forced the development of several methods able to tailor their spectral characteristics for specific applications. Basically, two different approaches can be adopted to specialize the grating spectra: one is based on complex grating profiles induced directly at the fabrication stage. Another, more interesting, approach relies on post processing methodologies that introduce localized defects along the grating, breaking its periodic structure. The presence of the defects leads to the formation of allowed bands or defect states within the grating bandgap. The introduction of finer scale spectral features results in new interesting perspectives in both telecommunications and sensing fields. In addition, the possibility to accurately control the defect states spectral features - depth, bandwidth, and spectral position - could allow the complete engineering of the FBG bandgap, opening the way to the realization of several new advanced and attractive photonic devices. This chapter is focused to review the main advancements in FBGs post processing to easily control the spectral features of the final device for specific applications.