Helix TWTs have always been known for their octave bandwidth capabilities, but are often thought of as having low peak power, low average power, and low efficiency. New developments in helix circuit design are changing this image. Helix TWTs are now capable of the following performance in PPM focused, conduction cooled formats: Peak powers of up to 50 kilowatts; average powers of 1.5 kW at 6 GHz; bandwidths of more than 2 octaves; overall effficiencies of 50%; frequency coverage to 50 GHz. This paper will describe the helix circuit features that make such performances possible: Oscillation suppression techniques, including pitch steps and resonant loss to permit high peak power operation; brazed helix-support post assemblies to increase average power; dispersion reducing loading shells to extend bandwidth; multistage depressed collectors to give high efficiency; miniaturized helix circuits to permit operation up to 50 GHz. Examples of these new techniques developed by various companies will be presented.