Evidence for the energetics, efficiency, and magnetic fields from BH angular momentum transport

I will review what observations can tell us about the energetics and magnetic field structures in jets that are produced by black hole angular momentum transport. These show that this must be a very efficient process, which must put tight constraints on the ``machine'' that converts gravitational to magnetic energy. I also discuss limitations in the current observational diagnostics, which are mostly in the radio, and prospects for overcoming some of these in the near future.