The most outstanding development in modern science was the foundation of quantum mechanics. Nevertheless, it is well known that most of the quantum effects only appear in the nature on the atomic or subatomic scale. However, a superfluid (characterised by its unusual properties, such as, zero viscosity, infinitely high thermal conductivity, existence of quantum vortices, etc.) represents the effects of quantum behaviour of particles on the bulk properties of a matter and so, superfluidity is, rightly, termed as macroscopic quantum phenomenon. Recently, however, the high resolution spectroscopy of doped molecules in 4^He nanodroplets (number of 4^He atoms, N ~ 10^3 – 10^4) and clusters (4^He¬_N–M, M being the doped molecule and N = 1, 2, ...) provided a unique method to explore superfluidity in such a microscopic system (MICROSCOPIC SUPERFLUIDITY), also. Numerous studies on such high resolution spectroscopy have been reported. Yet, no theoretical approach could give a sound explanation of the experimental findings, so far. However, in this book, we have given an appropriate answer to these, based on Macro-Orbital theory and also provided explanations for some other related aspects.