Review. The path to fusion power 1099 over 30 per cent of the cost of a fusion reactor. A measure of the efﬁciency of conﬁnement is the ratio between plasma and magnetic pressure β = P/(4B2), (6.3) where, as before, P is in atmospheres and B in tesla. Since the fusion power is proportional to β2B4 (see equation (3.4)) achieving high β is critical to large fusion power and thereby commercial success. However, when β exceeds a critical value the plasma is unstable and disrupts rapidly—these instabilities are understood and accurately predicted by theory. Shaping the magnetic conﬁguration and rotating the plasma have increased the achievable β. The recently developed spherical tokamaks (where the ratio of minor to major radius is approx. 1) achieve remarkable β values. For example, JET achieves β values of a few percent; MAST, the spherical tokamak at Culham, can achieve β’s of more than 30 per cent; ITER will operate at β of 4–5%. When the plasma is burning at high gain the energy needed to sustain plasma is negligible. However, for continuous operation the current must be sustained against resistive decay. For efﬁcient fusion it is essential that the external power (PCurrent) needed to drive the current be small compared with the fusion power. Culham researchers (Bickerton et al. 1971) showed that if the plasma pressure (β) is sufﬁciently high the current could in principle be sustained by a collisional effect—this they termed ‘bootstrap’ current. The bootstrap current was ﬁrst measured in a tokamak in 1988 (Zarnstorff et al. 1988). It is important to maximize the bootstrap current since it requires no external power. Current has been sustained indeﬁnitely in several experiments with a combination of bootstrap and radio frequency or beam current drive (Becoulet & Hoang 2008). The fraction of the current driven by the bootstrap often exceeds 50 per cent and can reach 100 per cent (Becoulet & Hoang 2008). ITER is expected to approach steady state with a partially bootstrap-driven current in its so-called hybrid modes. We can be rather conﬁdent that ITER (see below) will reach the burning plasma region indicated in ﬁgure 3. The extrapolation from JET to ITER is generally modest: the ITER operating temperature and β are regularly achieved in JET. However, the conﬁnement time must increase from less than 1 s to 3–4 s. ITER is twice as big as JET in every dimension and the magnetic ﬁeld is increased from approximately 3–4 T to 5 T. The semi- empirical scaling laws that interpolate rather accurately between results from machines with very different sizes, magnetic ﬁelds and plasma currents yield an ITER conﬁnement time of 4 s—slightly less than the random walk result (τE ∝ L3B2T 3/2). Negative developments are of course not excluded in the future. There could be new instabilities in the burning plasmas that will be studied, for the ﬁrst time, at ITER. An obvious candidate is the excitation of Alfvén waves triggered by slowing alpha particles when they pass through the Alfvén velocity. But theoretical and experimental simulations suggest that this is very unlikely. Tokamaks promise to scale to commercial reactors. Nonetheless, two alternative magnetic conﬁgurations are being pursued: the spherical tokamak and the stellarator. The spherical tokamak has two key advantages over the Phil. Trans. R. Soc. A (2010)