Increased knowledge concerning the levels of estrogen in normal and abnormal reproductive physiology has been made possible because of significant development in the chemical determination. But present day chemical estrogen measurements are still based upon the general principles described by Brown of hydrolysis, extraction, purification, fractionation, and finally, quantitative estimation. After the used of numerous modifications of colorimetry, fluorometric quantitative estimation has been developed and increased in sensitivity. Still further specificity and sensitivity was obstained by the use of thin layer chromatography, gas chromatography, and radioisotopic tracers. The gradual combination of these methods has brought the sensitivity of some of the techniques to two nanograms of estrogen, sufficient for the estimation of estrogens in the plasma of nonpregnant women. The use of gas chromatography appears to be most promising from the point of view of sensitivity and rapidity, but for the assessment of estrogen levels in nonpregnant females, gas chromatography must be preceded by thin layer. On the other hand, bioassay methods, such as the use of rat uterine weight method or intravaginal assay method, have served reasonably well and continues to be so at the present time for most clinical problems.