Luminescence is a phenomenon in which materials emit various radiations under various excitation techniques. The emission can be in the ultraviolet, visible, or infrared spectrum. Luminescence plays a vital role in various fields, from material science to biology, facilitating the optoelectronics industry and medical sciences. In this chapter, the mechanism of luminescence, types of luminescence, and applications of luminescence are explained. Firstly, types of luminescence, including photoluminescence, electroluminescence, bioluminescence, chemiluminescence, and thermoluminescence, are discussed. The chapter is followed by an explanation of the mechanism of photoluminescence, including upconversion and downconversion photoluminescence phenomena. Finally, the chapter concludes with the applications of luminescence in various fields.