This chapter gives an overview of photoluminescence, explaining the basic principles by its theories and through several approaches, such as steady-state and time-resolved photoluminescence, which depict the latest improvements and enhancements in the sensitivity, resolution, and fields of application of PL measurements. PL applies to a very large variety of materials: semiconductors, nanomaterials, organic molecules, and hybrid perovskites. The dynamics of excitations, as well as defect states, and the bandgap energies, are very important parameters of semiconductor research that need to be derived for the maximum efficiencies of electrical and optoelectronic devices. Information about size-dependent properties and knowledge about surface states on nanomaterials like carbon nanotubes, quantum dots, and nanowires are provided by PL, and it offers unprecedented opportunities at the frontier of science for the development of nanoscale sensors, light-emitting devices, flat panel displays, and biomedical imaging tools in nanotechnology. The utilization of PL enables the pharmaceutical industry to characterize different aspects of drug delivery systems, the interactions of biomolecules, and biosensor development. For instance, organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs) in organic electronics can optimize the luminous properties of organic materials with the help of PL analysis. The environmental sector takes advantage of PL in pollution identification and measurement, water quality assessment, and the development of novel materials for environmental remediation. Thus, photoluminescence provides an extremely powerful and versatile tool that is important for most fields of science. In such a context, this chapter seeks to underline the critical role of PL in pushing the boundaries of scientific research and industrial applications by focusing on the major ways of its usage and demonstrating the impacts on material characterization, product development, and technological innovation.