Defect levels in upconversion phosphors play a significant role in its applicability as a suitable luminescent material. Thermoluminescence (TL) is a defect-related phenomenon and considered as an inevitable tool to understand the nature and distribution of traps/defects in the crystal lattice. This chapter reviews the physics of the thermoluminescence related to dosimetric applications in upconversion materials. Enormous research has been carried out to comprehend the fundamental mechanism and improve the materials characteristics as well as to evolve advance and novel thermoluminescence materials. Nowadays, thermoluminescence dosimetry (TLD) is a well-known dosimetric technique with a wide range of applications in personnel, environmental and clinical dosimetry. TLD is based on the materials which are previously exposed to ionizing radiations, emit visible radiations, when they are heated. The impurities in the upconversion TL materials encourage localized energy levels within the forbidden energy band gap which plays a vital role in TL process. By detecting the presence and nature of these defect levels, the sensitivity of TL is unrivalled. This chapter focuses on the investigation into the relation between the luminescence and defects involved in the process. Moreover, various models to understand the TL properties in the upconversion materials through energy band theory are discussed in detail. Different types of radiation exposure employed in TL studies display a remarkable impact on the trap distribution resulting in shallow and deep traps. Various techniques to determine different trapping parameters such as activation energy, order of kinetics, and frequency factor are discussed in detail which provides a comparative framework to interpret many observations. The knowledge of kinetic parameters namely order of kinetics equips one to understand recombination process and trapping profile comprehensively. The present chapter also discusses the behavior of TL glow peaks on the basis of order of kinetics which indicates the trapping/retrapping of charge carriers resulting to TL emissions in the phosphors.