The pieces of the jigsaw puzzle of volume homeostasis in human pregnancy are being put together gradually. This chapter has focused on recent advances in our understanding of factors controlling extracellular fluid volume in normal pregnancy and their disturbance in women who develop pre-eclampsia. We have explored the clinical implications of these guidelines for management of patients with pre-eclampsia. Clearly there is still much to be learned. Studies of the cellular and subcellular handling of sodium are still in their infancy and will add much to our understanding of the physiology of volume homeostasis in normal pregnancy and its disturbance in pre-eclampsia and other causes of hypertension in pregnancy.