The phenolic fraction of olive oils can be analyzed in different manners: the simple total quantitative determination is carried out by a colorimetric method, and the single phenols qualitative and quantitative analysis by using chromatographic or electrophoretic methods. Many methods have been set by high-performance liquid chromatography (HPLC); the quantification is simple and derivatizing reagents are not necessary, as in gas chromatography. GC methods do however have some advantages with respect to HPLC: lower detection limits and better separations. Hyphenated methods like GC-MS or GC-MS/MS add a direct qualitative information and further sensitivity due to ion current analysis or selected ion monitoring. Even trace compounds can be detected and identified with this technique; the aglycons of ligstroside and oleuropein can be detected too; only the whole glycosides, ligstroside and oleuropein, cannot be detected in GC. The importance of the analysis of the phenols is due to their antioxidant action, but their amount depends on many factors, like the growth and maturation of the olives, the oil preparation, the variety and the climate. The determination of low-level free phenols in olive oils is an aid to evaluate differences among samples from different varieties and some compounds can be related to the autoxidation stages of olive oils. This chapter reports the chromatographic and spectroscopic approach to GC-MS and GC-MS/MS analysis and the results obtained on 76 Sicilian monovarietal virgin olive oils (six different cultivars), produced in the years 1998-2000.