I briefly review the observational evidence for a small cosmological constant at the present epoch. This evidence mainly comes from high redshift observations of Type 1a supernovae, which, when combined with CMB observations strongly support a flat Universe with . Theoretically a cosmological constant can arise from zero point vacuum fluctuations. In addition ultra-light scalar fields could also give rise to a Universe which is accelerating driven by a time de- pendent -term induced by the scalar field potential. Finally a dominated Universe also finds support from observations of galaxy clustering and the age of the Universe. The cosmological constant was introduced into cosmology by Einstein in 1917. Ein- stein, under the influence of Mach's principle, constructed a Universe which was static and closed - a configuration that could only arise under the joint influence of both mat- ter and a cosmological constant. Subsequently Friedmann derived expanding solutions to the Einstein equations and Einstein acknowledged that the introduction of was probably unnecessary, particularly in view of Hubble's discovery that the Universe was expanding. However, interest in the cosmological constant remained, partly due to the rich variety of new solutions which arise in the Einstein equations with a cosmological constant includ- ing: the static Einstein Universe, singularity free 'bouncing' models, quasi-static 'loitering' models etc. Interest in was reignited in the late 1960's, when it was felt than an excess of QSO's was being observed at redshift 1.95. This observation was difficult to explain in the framework of standard FRW cosmology, but easier to account for if the Universe loitered at that redshift. More recently a large cosmological constant at an early epoch is the basis of the inflationary model, and a much smaller cosmological constant at a much later epoch is suggested by current observations.