Arguments favouring > 0 at the present epoch essentially stem from four sets of observations:

(i) *The age issue:* A high value of the Hubble constant
*H*_{0} ~ 80 km/sec/Mpc predicts a short age of the
universe which is incompatible with the ages of the oldest stars (12 -
16 Gyr) unless the universe is open
(_{m} <
0.1) or flat and
dominated _{m} +
_{} = 1.
The appeal of this argument has somewhat decreased following
recent Hipparcos parallax measurements indicating a lower value
*H*_{0}
67 km/sec/Mpc and also a lower age for globular clusters:
11.5 ± 1.5 Gyr. Still, recent observations of
old galaxies at high redshifts
are extremely difficult to accommodate
within the framework of a flat matter dominated cosmology unless
the Hubble parameter is very small (*H*_{0}
45 km/sec/Mpc;
section 4.1).

(ii) *Structure formation:* The standard COBE normalized
cold dark matter model of structure formation with
_{m} = 1
appears to be in serious conflict with observations.
The situation may be remedied if the universe is flat, with most of matter
smoothly distributed in the form of a cosmological constant and only
a small fraction
_{m}
*h* 0.2 in
clustered matter.
(Here *h* is the Hubble constant in units of 100 km/s/Mpc).
Studies of the abundance and evolution of clusters of galaxies and of
lensing by clusters also appear to favour a low density universe
(section 4.6).

(iii) *Baryon excess in clusters:* In a spatially flat universe with
_{m} = 1 the
mass fraction in baryons in the Coma cluster is expected to greatly
exceed nucleosynthesis bounds
leading to what has been called
the `baryon catastrophe'. The mass fraction in baryons can be kept in
agreement with nucleosynthesis constraints only if
_{m}
*h* 0.16
[210]
(_{m}
includes contribution from baryons and clustered dark matter).
Agreement with the Inflationary scenario which strongly favours a
spatially flat universe then suggests that the remaining
mass might be in the form of a cosmological constant.

(iv) *High redshift supernovae and the cosmic microwave background:*
Preliminary results
from this rapidly advancing field of cosmology
suggest that the universe may be accelerating universe with a dominant
contribution to its energy density coming in the form of
cosmological -term.
These results, when combined with CMB anisotropy observations on
intermediate angular scales, strongly support a flat universe
_{m} +
_{} = 1 with
_{} ~ 0.6 - 0.7
(sections 4.3 &
4.4).

In the first half of this paper we shall briefly review the present observational status of the cosmological constant referring the reader to the original papers and earlier reviews [26, 37] for more details.