An experimental and theoretical research program on the ubitron/FEL is in progress at the Naval Research Laboratory. The theoretical program includes the nonlinear analysis of two specific configurations: a helical wiggler/axial guide field in combination with a cylindrical waveguide, and a planar wiggler in combination with a rectangular waveguide. The analysis includes the description of the injection of the beam into the wiggler, wiggler inhomogenieties, efficiency enhancement by means of tapered wigglers, beam thermal effects, and harmonic interactions. The planar wiggler model includes parabolic pole faces for enhanced focussing of the beam. Operation of a 13-18 GHz ubitron/FEL with a helical wiggler/axial guide field configuration is reported. The experiment employs a 190-250 keV electron beam with curents up to 100 A. Small signal gains as high as 17-19 dB have been observed, and are in reasonable agreement with the simulations. The initial design work on a third harmonic experiment has begun based upon a planar wiggler/rectangular waveguide configuration. Because the effect of a tapered wiggler has been shown to reduce the influence of beam thermal spread, issues relevant to the design of a tapered wiggler harmonic experiment are discussed.