Platelet-activating factor (PAF) is a potent mediator that occurs at very low concentrations in cells and tissues. Accurate quantitation of PAF has always been difficult because of the physicochemical properties of PAF and its structural similarity to several much more abundant phospholipids. Numerous assays for PAF have been developed, all of which have their strengths and limitations. Herein, this chapter describes a high-pressure liquid chromatography (HPLC)-tandem mass spectrometry assay for PAF. Major strengths of the method are its sensitivity (detection limit = 1 pg) and selectivity. Another advantage is that, by using liquid instead of gas chromatography, sample derivatization is avoided. The limitations of the method are its use of expensive instrumentation and the requirement of performing two HPLC runs per sample. Detailed technical advice on application of the method to various types of samples is given.