Intracore fiber Bragg gratings have been extensively used as longitudinal strain sensors both bonded and embedded in numerous applications, fulfilling the same task as conventional resistive strain gages. Comparative results obtained in composite laminates with both types of sensors show an excellent correlation in those places where the strain distributions are smooth. In this case, optical sensors offer additional advantages: unnecessary calibration, multiplexing capability, small size, embedding ability, etc. But optical sensors reveal all their potential in those locations where the anisotropy of the composite structural element promotes strong strain distributions, and complex stress fields. In these cases, the analysis of the distorted spectrum of a grating submitted to an intense strain gradient offers a big amount of information, even without using spectrum integration methods. Furthermore, the knowledge of the optical behavior of fiber Bragg gratings submitted to transverse loads allows having additional information about the residual stress field promoted during the manufacturing process, and the stress release due to the machining of the part. This paper demonstrates theoretically and experimentally that fiber Bragg gratings can be valuable tools not only to monitor complete composite structures in service, but to analyze the stress and strain state of those particular configurations in which any kind of information, due to their complexity and high requirements, is essential.