Upconversion phosphors, typically doped with rare earth ions, are capable of converting multiple lower energy photons into a single higher energy photon. Owing to their photostability, sharp emission bands, anti-stokes emission, and long luminescence lifetime makes them promising candidates for applications in bio-imaging, opto-electronics, solid-state lighting, and anti-counterfeiting measures. However, optimizing these materials for such applications requires a detailed understanding of structure–property relationships. This chapter provides a comprehensive review on the spectroscopy techniques aimed at characterising distinct properties of the upconversion phosphors. The discussion is structured around six characterisation goals which includes (a) analysis of electronic structure using PL and UV–Visible, (b) investigation of surface ligands bound to the upconversion phosphors using IR and NMR, (c) examination of vibrational modes using FTIR and Raman, (d) determination of elemental composition using XPS, EDXS, and EELS (e) evaluation of local atomic environment around dopant ions using XAS, and (f) assessment of luminescence lifetime dynamics using TRS. By correlating spectroscopic data with optical properties, this chapter highlights the role of spectroscopy as a vital role in the rational design and performance enhancement of upconversion phosphors.