  We describe the current status of the Ca optical frequency standards with
laser-cooled neutral atoms realized in two different laboratories for the
purpose of developing a possible future optical atomic clock.
  Frequency measurements performed at the Physikalisch-Technische Bundesanstalt
(PTB) and the National Institute of Standards and Technology (NIST) make the
frequency of the clock transition of 40Ca one of the best known optical
frequencies (relative uncertainty 1.2e-14) and the measurements of this
frequency in both laboratories agree to well within their respective
uncertainties.
  Prospects for improvement by orders of magnitude in the relative uncertainty
of the standard look feasible.
