Distinguishing a single molecule from an ensemble requires a detailed understanding of a three-level system. The phenomenon underlying the emission from a single molecule depends on a number of factors, such as transition probabilities, emission rates, and selection rules associated with the energy levels in a three-level system. Among other factors, the quantum jump between two states (fluorescence and dark state) is a strong indicator of a single molecule. Such jumps cause a sudden burst of photons after a period of darkness at regular intervals (often equal to the lifetime of a molecular state such as, triplet state). Another key aspect elucidated by a single molecule is photon correlation, which occurs when the system jumps between singlet and triplet states, causing a strong correlation (during the ON cycle) or no photons for a brief period (OFF state). This is better illustrated for Pentacene molecule that exhibits all the phenomenons associated with a single molecule. As an example, Pentacene molecule is considered for case study. Infact, the last section is dedicated to the study of Pentacene molecule that elucidate key aspects of a single molecule.