Few fields have advanced faster over the past quinquennium than separation and estimation of steroids by the technique of gas phase chromatography. A detailed and complete review of this topic would therefore be beyond the scope of the authors contributing to this monograph. It was, however, felt that a discussion of some of the highlights of this rapid advance might be of help for laboratories estimating steroids in biological samples. One of the difficulties in producing a monograph of this kind is that before it can appear in print it is likely that some of the methods it discusses will have been overtaken by better methods, so swiftly is progress now made. No editorial power has been exercised in trying to make a uni form account of technology in this field, and the idiosyncrasies of each individual author have been left intact. Through this approach we hope that what has been lost in scholarly appearance is regained in general appeal.