Some of the main plasma physics challenges associated with achieving the conditions for commercial fusion power in tokamaks are reviewed. The confinement quality is considered to be a key factor, having an impact on the size of the reactor and exhaust power that has to be managed. Plasma eruptions can cause excessive erosion if not mitigated, with implications for maintenance and availability. Disruptions are a major concern – one large disruption could terminally damage the reactor so it is important to understand the loads they impart to the structure, and put in place appropriate protection and an effective avoidance/mitigation strategy. Managing the exhaust of heat and particles from the plasma is likely to be a significant issue, which may be mitigated if an advanced confinement regime can be identified. The advanced divertor structures that may be required to handle the exhaust have a significant impact on the design of a fusion reactor. Three strategies can be identified to take account of the physics challenges, with different implications for the timescale to fusion power: (1) a staged approach with the size of each step determined by our confidence in the predictive capability of our models; (2) a single, big step with contingency built into the design where possible to accommodate the uncertainty in physics predictions, and (3) a single big step with optimistic physics assumptions and no contingency, accepting the increased financial and reputational risk that comes with such an approach.