This chapter offers an integrated view of the refractive indexRefractive index in rare-earth (RE)-doped glasses, bridging electromagnetic foundations, measurement techniques, and dispersion modeling. It starts from the Drude–Lorentz formalism for the complex dielectric response and its relation to (n) and (k) and discusses a fractional derivative extension to capture structural disorder and non-trivial damping typical of amorphous matrices. Experimentally, it mentions routes to obtain n(λ), highlighting prism couplingPrism coupling as a non-destructive, high-precision option for thin films and waveguides. The chapter then reviews and applies dispersion models (SellmeierSellmeier & Wemple–DiDomenico dispersion, Cauchy, Herzberger, exponential, and Wemple–DiDomenico) to extract physically interpretable parameters and to predict n(λ) outside measured windows. Throughout, it shows how RE species and concentration modify local polarizability and shift resonances, enabling fine design of photonic devices across the visible–NIR.