This chapter introduces absorption spectroscopyAbsorption spectroscopy as a foundational probe of light–matter interaction and a workhorse for characterizing rare-earth (RE)-doped glasses. It opens by defining absorption spectra and reviewing the historical and theoretical basis—Beer–Lambert law and practical spectrophotometry—linking measured absorbance to material composition and electronic structure across the UV–Vis–NIR range. Emphasis is placed on intra-4f transitions of RE3⁺ ions: despite being nominally dipole-forbidden, crystal-field-induced parity mixing renders weak but resolvable absorption bands whose intensities scale with dopant concentration until clustering or energy-transfer effects emerge. Practical examples (e.g., Er3+ in tellurite Zn–Na glasses) show linear growth of the 4I15/2 → 2H11/2 band with concentration, evidencing increased density of optically active centers in the studied regime. Beyond discrete 4f bands, the chapter uses Tauc analysis to determine direct/indirect optical bandgapsBandgap and discusses Urbach energyUrbach energy as a metric of disorder, showing that RE incorporation narrows the bandgap and raises Urbach energyUrbach energy—trends with direct implications for balancing transparency, non-radiative losses, and broadband functionality in photonic devices.