The mechanism of upconversion is well known since 1960s but in past two decades, a wide variety of upconversion materials has been investigated by tuning shape and size of materials. Indeed, many researchers paid a huge attention specifically on rare earth-based phosphor materials due to their interesting properties to utilize for real time applications. This chapter is opted to provide a general introduction of up conversion phosphor materials dealing with different upconversion mechanisms such as excited state absorption, energy transfer, ground state absorption, and photo avalanche. Moreover, a brief overview also discussed fruitfully on charming applications of up conversion phosphors such as bio-imaging, photodynamic therapy, latent fingerprint detection, temperature sensor and security links.