  The importance of diquarks has been noticed in the context of several
elementary particle processes. In recent years the study of diquarks has become
of considerable interest in highlighting its role in high density physics from
the point of view of astrophysical situations and cosmological conditions. In
the present paper an attempt is made to briefly review the role of diquarks in
conventional physics and in Quantum Chromodynamic phase transition. The
importance of equation of state for point-like scalar diquark (PSD) and
extended scalar diquark (ESD) gas and matter is reviewed. In this regard, the
sensitivity of ESD energy on the effective interaction parameter, within the
frame work of \phi^4 theory, is studied. The role of diquarks in astro-physical
situations and cosmological conditions with reference to the early phases of
the origin of the universe is discussed. The recent discovery of \theta^+
(1540) baryon - a pentaquark, which may be considered as a two highly
correlated ud-diquark pairs and an anti strange quark, is also highlighted.
