In this chapter, the main findings of spectroscopic analysis applied to epoxy/thermoplastic (TP) blends are presented. This includes vibrational spectroscopy (infrared and Raman), nuclear magnetic resonance spectroscopy, fluorescence spectroscopy, energy-dispersive X-ray spectroscopy, and dielectric relaxation spectroscopy. These techniques have been employed alone or combined to monitor polymerization processes, the glass transition, and structural relaxations, to study reaction-induced phase separation (RIPS), to obtain a complete chemical characterization of the different phases, or to analyze compositional gradients at the interphases. The study of the specific interactions between the epoxy components and the thermoplastic modifier allowed the interpretation of chain interdiffusion and miscibility data and helped to explain microstructure. Several examples are discussed.