Insertion loss measurements of waveguide components have been made with an accuracy 0.2 dB at the 10-dB level and 2.8 dB at the 30-dB level. Attenuation measurements of 60-mm TE10circular waveguide have been made over the frequency range 33 to 110 GHz with total uncertainty of 0.4 dB. Intercomparison between calorimeters developed by national laboratories at 100 GHz resulted in differences less than 0.5 percent. In measuring optical fibers calorimetry was used to measure loss and power with an accuracy of 1 to 2 percent. Loss measurements by comparison with a standard have resulted in accuracies of 0.5 dB in losses of 40 dB/km in the 1.0- to 1.6-µm range. Shuttle pulse measurement of pulse spreading indicates that pulsewidths of 0.4 ns are increased to 4.0 ns by passing through 2 km of fiber at a wavelength of 0.9 µm. Interferometer techniques were developed for determination of the complex permittivity of liquids and solids over a wide temperature range in the frequency range from 10.0 GHz to 18 THz. Complex permittivities have been measured at 94 GHz by transmission through a dielectric slab. Errors reported in relative permittivity and loss tangent are 0.2 and 2.5 percent, respectively.