Pre-eclampsia is a multisystem disorder, of unknown aetiology, usually associated with raised blood pressure and proteinuria. Although outcome for most women and their babies is good, it remains a major cause of morbidity and mortality. A wide range of interventions for prevention and treatment of pre-eclampsia have been evaluated in randomized trials. This evidence provides the basis for a rational approach to care. Overall, there is insufficient evidence for any firm conclusion about the effects of any aspect of diet or lifestyle during pregnancy. Antiplatelet agents are associated with a 19% reduction in the risk of pre-eclampsia (relative risk 0.81; 95% CI 0.75, 0.88), a 7% reduction in the risk of preterm birth (RR 0.93; 95% CI 0.89, 0.98), a 16% reduction in the risk of stillbirth or neonatal death (RR 0.84; 95% CI 0.74, 0.96) and an 8% reduction in the risk of a small for gestational age baby (RR 0.92; 95% CI 0.85, 1.00). For mild to moderate hypertension, trials evaluating bed rest are too small for reliable conclusions about the potential benefits and hazards. Antihypertensive agents halve the risk of progression to severe hypertension (RR 0.52; 95% CI 0.41, 0.64), but with no clear effect on pre-eclampsia (RR 0.99; 95% CI 0.84, 1.18), or any other substantive outcome. For severe hypertension, there is no good evidence that one drug is any better than another. Plasma volume expansion for severe pre-eclampsia seems unlikely to be beneficial, although the trials are small. The optimum timing of delivery for pre-eclampsia before 34 weeks is unclear. Magnesium sulphate more than halves the risk of eclampsia (RR 0.41; 95% CI 0.29, 0.58) and probably reduces the risk of maternal death (RR 0.54; 95% CI 0.26, 1.10). It is also the drug of choice for treatment of eclampsia.