This book presents the fundamental and applied properties of different phosphor materials that exhibit the phenomenon of persistent luminescence (PerLum). PerLum offers a solution for the limitations of existing fluorescent probes, with new lanthanide-based materials for in vivo bio-imaging and demonstrates the practical applications of these materials. The book offers a basic understanding of PerLum, different mechanisms, models, related spectroscopic techniques, and a few applications essential for anyone interested in learning about PerLum materials. This is partly due to the fact that the property of long-lasting luminescence is closely associated with other properties and functionalities, which besides being essential for many technological applications, such as bioimaging and biosensing, involve in-depth physics and chemistry. Although the concept of luminescence has been explored for many decades, the different types of luminescence have always led to versatile breakthroughs in different fields of science and technology. Among the same is one more well-established phenomenon known as "PerLum," which is characterized by the maintenance of luminescence for periods ranging from minutes to hours in the absence of an external excitation source. Because of their remarkable properties, an enormous number of PerLum materials have been developed and are now widely used in a variety of fields, particularly bioimaging, biosensing, therapeutics, etc.