AbstractFollow-up of unidentified Fermi sources has expanded the number of known galactic-field “black widow” and “redback” millisecond pulsar binaries from four to nearly 30. Several systems observed by Chandra, XMM-Newton, Suzaku, and NuSTAR exhibit double-peaked X-ray orbital modulation. This is attributed to synchrotron emission from electrons accelerated in an intrabinary shock and Doppler boosting by mildly relativistic bulk flow. We briefly discuss the rich complexity of these systems, their astrophysical utility, and open questions.