Pre-eclampsia is a common condition, unique to pregnancy, and almost unique among medical conditions in that we have, at best, only a patchy knowledge of the underlying aetiology and pathophysiology. In this chapter, established and new hypotheses will be discussed and the recent supporting evidence for each will be reviewed. The principal systems affected by pre-eclampsia, namely the cardiovascular, coagulation, liver and renal systems, together with cerebral manifestations of this disease will be discussed in turn.