This paper describes methods by which the over-all efficiency of traveling-wave tubes can be increased through a considerable reduction in collector-element dissipation without appreciable deterioration of rf performance. Various methods are described which permit depression of collector potential considerably below the synchronous helix voltage. In these methods, suppression of secondary-electron emission is necessary to remove limitations imposed by the increased helix dissipation. The use of high-resistance coatings, a hollow suppressor electrode, magnetic shielding, and other devices for inhibiting secondary emission is discussed.