Chromatography is widely used for qualitative and quantitative analysis of phytochemicals and biological samples. Gas chromatography (GC) measures complex mixtures of volatile substances, which vaporize without decomposition. It is also a widely used instrument for analysis of solid, liquid and gaseous samples. This technique can provide information about the qualitative and quantitative composition of both inorganic and organic analytes in complex mixtures, their molecular structures, isotopic ratios of atoms in samples and the structure and composition of solid surfaces. The method has been a key analytical technique in the food industry. It enables complex organic substances to be separated and identified quickly and cheaply. Many nonvolatile substances such as amino acids, steroids and high molecular weight fatty acids can be derivatized to form volatile substances that can be separated by GC. This book describes biological applications of gas chromatography method for several phytochemicals found in plants, foods, body metabolites, and elsewhere, with emphasis on those where biological activity (injury or health benefits) has been reported.