Spontaneous transitions from the low ‘‘L-mode’’ to high ‘‘H-mode’’ of tokamak plasma confinement, first observed during neutral beam heating experiments on ASDEX, are now routinely achieved in many tokamak experiments. The H-mode regime is attractive as it offers the possibility of enhanced confinement, and thus a route towards a more ‘‘compact’’ and cost-efficient fusion power-plant. Transition to H-mode is now routinely achievable in the Mega-Amp Spherical Tokamak (MAST) [A. C. Darke et al. , Fusion Technology 1994 ~ Elsevier, Amsterdam, (1995), Vol. 1, p. 799] for both Ohmically and neutral beam injection (NBI) heated plasmas (P NBI ~ 0.5–1.7 MW). H-mode plasmas can be either center stack limited or X-point diverted, exhibiting regular Type III edge localized modes (ELMs). Global confinement in H-mode with low frequency ELMs is consistent with the international IPB(y,2) scaling and exceeds the scaling by a factor ~1.5–2.0 for high performance discharges. Confinement degrades with increasing ELM frequency (which in turn scales with power and density) as for conventional tokamaks. Densities above the Greenwald limit (G~1) have been achieved for plasma currents up to 0.8 MA using gas-fueling, and up to 0.9 MA using a low field side multi-pellet injector. High field side fueling, on the other hand, can be supplied via a gas-feed located at the center-column mid-plane, this technique having been found to dramatically enhance H-mode accessibility and quality. When combined with Connected Double Null plasma topology, a significant reduction in Ohmic L–H power threshold can be achieved; as a result, power threshold data are now in broad agreement with a number of the latest scaling law predictions. Following the transition to H-mode, power crossing the inner separatrix remains low, resulting in a high recycling scrape-off layer (compared with partial detachment in L-mode). To date, with NBI power limited to 1.7 MW, H-mode MAST plasmas have shown no evidence of having approached a beta limit. High performance H-mode discharges are at sufficient poloidal beta, however, to enable the first studies of the Neoclassical Tearing Mode, the MHD instability responsible for limiting the achievable beta in conventional tokamaks.