Using the outside launch with vertical polarization, electron cyclotron heating (ECH) experiments in DIII-D were carried out at 60 GHz at fundamental and second harmonic frequencies. The results are consistent with wave absorption theory for accessibility. For fundamental heating with a cut-off layer between the launch location and the resonant surface, good absorption efficiency was obtained. A process for mode conversion at the wall from the extraordinary mode to the ordinary mode is postulated to explain this result. For most efficient heating it was important to avoid the presence of a locked mode. For low density plasmas at the fundamental, the global energy confinement exceeds Kaye-Goldston scaling by about a factor of 1.4. However, heating at the second harmonic agrees numerically with the Kaye-Goldston scaling formula, for ECH alone or for ECH combined with neutral beam injection (NBI), and shows a mass dependence consistent with TE ∼ √Ai. The global energy increase was insensitive to the resonance location for qres < 2–3. Comparing data at both harmonics in the low density range, where the energy confinement of Ohmic plasmas satisfies neo-Alcator density scaling and Prf/Pohm > 1, global energy confinement scales as TE ∼ Bt−0.3. Since plasma current and plasma geometry were not varied for these discharges, a scaling with safety factor q is also consistent. Using ECH alone, H-mode confinement was achieved for second harmonic heating in the central core of the plasma at a power threshold of ∼0.75 MW for plasma parameters Ip = 0.5 MA, and e = 1.1 × 1019 m−3. The energy confinement times during the H-mode are similar to Ohmic values with a normalized confinement time τE/Ip ∼ 200 ms/MA. For ECH and NBI H-mode plasmas, the transition from the L-mode to the H-mode was correlated with reduced magnetic fluctuations in the divertor region. For ECH H-mode plasmas, transport analysis shows that (1) the H-mode obtained with ECH is accompanied by an improvement of the electron thermal diffusivity χe relative to the L-mode over at least part of the plasma, (2) the ion thermal diffusivity χi is larger than predicted by neoclassical theory for the L- and H-modes, and (3) χe is approximately equal to χi for the ohmically heated discharge and for the L-mode and H-mode discharges heated by ECH discussed in the paper.