If the VLBA target source is not sufficiently strong for self-calibration and/or if absolute positional information is needed, then VLBA phase referenced observations must be employed (Beasley & Conway 1995). A VLBA phase reference source should be observed frequently and be within a few degrees of the VLBA target region, otherwise differential atmospheric (tropospheric and ionospheric) propagation effects will prevent accurate phase transfer. In the short term, VLBA users can draw candidate phase calibrators from the MERLIN phase calibrator grid of Patnaik (1992) and Browne (1997), which will eventually be extended to cover the sky north of declination -30 degrees. A VLBA survey of these MERLIN calibrators is in progress, to determine which sources are compact enough to serve as good VLBA phase reference sources and to obtain improved reference source positions (Beasley 1996); preliminary results from this VLBA Calibrator Survey are available from the VLBA home page (see Section 26.3).