The fact that light exists in minuscule chunks of energy packets ( \(\sim \hbar \nu \) ) gives it a distinct character. It appears that light can exist or be prepared in a number of quantum states: Fock state, coherent state, squeezed state and numerous other states. In fact, there are instances where light can exist in a mixed state, i.e., a classical state with local quantum behavior. These states with bizarre quantum behavior have unique properties. Specifically, light in a specific quantum state (quantum entangled state or squeezed state) gives rise to precision measurement of quantities (phase or intensity) beyond the classical limit. In addition, entanglement can be exploited to improve contrast, resolution, and sensitivity. These peculiar properties have repercussions in precision optical microscopy. The quantum state of light is known to play a critical role in super-resolving features. In this chapter, we will discuss all these aspects, beginning with some of the basic quantum states of light and exploring its potential in optical microscopy.