A comparison of four slow-wave propagating structures for traveling-wave tubes is made. Recent experimental performance of several types of tubes using two of these circuits is given. Amplifier performance of helix tubes at 10,000 Mc, and wide-tuned oscillator performance (1.5 to 1) with second-harmonic output around 20,000 Mc are described. The analysis, construction, and performance of a 10,000-Mc disk-loaded-rod tube are presented. Apertured-disk and spiraled-waveguide structures are discussed briefly and compared with the helix and disk-loaded rod.