Alpha fetoprotein (AFP) is the major protein of fetal serum and most resembles albumen, which replaces it shortly after birth. It is produced by fetal liver and passes into the amniotic fluid (AF) via fetal urine. A small amount crosses the membranes into the maternal circulation. Excluding fetal blood contamination, elevated AF/AFP levels indicate fetal demise or one of several abnormalities. Maternal serum (MS) AFP measurement can be used as a screening procedure to identify neural tube defects providing a rigorous protocol is followed. This requires that a laboratory establish its normal range of MS/AFP levels between 15 and 20 weeks gestation, employ a reliable assay with adequate controls, and has recourse to genetic counseling as well as expert sonography, amniocentesis and amniography if necessary. Pregnancy is the only normal situation in which AFP is present after birth. It may be present in high levels in certain malignancies and has been useful in monitoring their recurrence. This article will review the history, biochemistry, and different assays of AFP in AF and blood as well as the indications and limitations for their use.