  This paper reports the unified microscopic theory of a system of interacting
bosons such as liquid $^4He$.Each particle in the system represents a $(q;-q)$
pair moving with a centre of mass momentum K.Particles form bound pairs below
$\lambda$-point and have a kind of collection binding between them.The binding
is idenified as an energy gap between the superfluid and the normal states of
the system.The $\lambda$-transition is a consequence of interparticle quantum
correlations.It follows an order-disoder of particles in their phase structure
as well as the onset of Bose_ Einstein condensatin in the state of $q={\pi}/d$
and K=0.In addition to the well known modes of collective motion such as
photons,rotons, maxons etc.,the superfluid state also exhibits a new kind of
quasi-particle,omon,characterised by a phononlike wave of the oscillations of
the momentum coordinates of the particle.The theory explains the properties of
$He-II$ at quantitative level and vindicates the two-fluid theory of Landau.The
paper finally describes the way this theory could help in understanding the
superfluidity of 1-D and 2-D systems.It also analyses the possibility of
applying this approach to develop similar framework for a fermion system
including an atomic nucleus.
