An important prerequisite for the management of high risk pregnancies is the accurate prediction of foetal lung maturity. A number of indices of foetal lung maturity based on the determination of surfactant constituents in the amniotic fluid have been proposed. Amniotic fluid contains phospholipids, including phosphatidylcholine (lecithin), sphingomyelin, phosphatidylinositol and phosphatidylglyerol (PG), some enzymes of the pathways of phospholipid synthesis, lamellar bodies, and lung specific apoproteins. The amount of these substances in amniotic fluid changes towards the end of gestation in a manner related to foetal lung maturity. Determination of the lecithin to sphingomyelin (L/S) ratio is by far the most widely used and accepted method. However, there is still controversy regarding the high incidence of false immature values, and the increased incidence of false mature values (from 1 to 15%) especially in pregnancies complicated by diabetes mellitus; an immature L/S ratio may predict respiratory distress syndrome (RDS) only in about 50% of cases. The incidence of false immature L/S ratio as well as other amniotic fluid tests depends upon patient variability, method employed, threshold taken for differentiating a normal from an abnormal condition, and on the fact that only few authors report their results in terms of sensitivity and specificity. Where laboratory facilities are minimal, it is advisable to perform the shake test or to measure the optical density of amniotic fluid. However, when these tests indicate immaturity, additional tests, such as determination of the L/S ratio or the lung profile (including PG), must be performed. The utilization of these tests is recommended for: 1) timing of delivery prior to elective caesarean section; 2) management of complicated pregnancies; and 3) recognizing indications for pharmacologic prevention of RDS in utero or at delivery.