  After a short history of the $\Lambda$-term it is explained why the
(effective) cosmological constant is expected to obtain contributions from
short-distance-physics, corresponding to an energy scale of at least 100 GeV.
The actual tiny value of the cosmological constant in any natural scale of
units represents, therefore, one of the deepest mysteries of present day
fundamental physics. We also briefly discuss recent astronomical evidence for a
cosmologically significant vacuum energy density causing an accelerating
expansion of the universe. This arises mainly from the Hubble diagram of type
Ia supernovae and from the observed temperature fluctuations of the cosmic
microwave background radiation. If this should become an established fact, we
are also confronted with a disturbing {\it cosmic coincidence} problem.
