The dynamics of a minimally coupled scalar field in the expanding universe is discussed with special reference to phantom cosmology. The evolution of the universe with a phantom field vis-a-vis a quintessence field is compared. Phantom cosmologies are found to have two special features i) occurrence of a singularity where the scale factor, the energy density and Ricci curvature scalar diverge to infinity. This singularity occurs at a finite time, depending on w during cosmic evolution, ii) degeneracy in the determination of w(z_m) for a particular transition redshift z_m which seems to impart similar observational properties to corresponding phantom and quintessence models and makes them compatible with the cosmological observations. Constraints over phantom fields are derived using recent WMAP data and the range of $w$ is narrowed down to -1.18 < w < -0.93 with the corresponding value of the deceleration parameter lying in the range - 0.8 < q_0 < -0.52. An observational test, based upon the observations of low redshift galactic clusters, is suggested to discriminate between the quintessence and phantom dark energy.

12 pages, revtex, 3 figures, 1 table, references added