Since the late 1980s, there has been a renewed interest in space-time adaptive processing (STAP) with applications to airborne early warning (AEW) radars, as illustrated by the fact that several major research and development projects have been initiated. This paper presents our observations on some of space-time processing algorithm research issues which need further attention. Discussed in certain detail are the suppression of coherent repeater jammers and the calibration error effect on the emulated airborne clutter data using the inverse displaced phase center array method. Other issues commented on in this paper include aperture design, system performance comparison, polarization-space-time adaptive processing, new applications, etc.