## 1 Introduction
Other compounds such as vitamins (tocoperlo), pigments such as \(\beta\)-carotene and lutein, phenolic compounds such as secoiridoids (derivatives based on hydroxytrosol and tyrosol structures) and flavones (apigenin, luteolin) and hydrocarbons such as squalene, a terpenoid hydrocarbon and precursor of the biosynthesis of steroids, are found in minor quantity; however, they are endowed with important biologic activity [2; 3]. Consequently, due to its rich composition in nutrients of unique nutritional value, many health claims have been made for its health-promoting properties [4; 5; 6]. Hence, a steady and rapid increase regarding its demand, inside and outside the Mediterranean basin, is observed. However, the high nutritional value, in combination with high production costs and high demand, makes olive oil a targeted product fraud. Therefore, economically motivated adulteration incidents and cases of fraudulence are continuously revealed. In order to avoid any kind of adulteration, the olive oil market is closely monitored by various organizations, which have suggested criteria and limits that should be respected by producers and distributors. Quality criteria including chemical composition parameters, organoleptic characteristics (aroma, taste, flavor), and guidelines of analysis methods have been proposed by the IOC and the Codex Alimentarius (CODEX STAN 33-1981). These proposed standards are not mandatory; however, they should be respected by the countries that agree to follow them. On the other hand, regulation of the European Community (EEC) No 2568/91 imposes compulsory parameters regarding quality and purity criteria, that should be adopted by the EU members. Furthermore, with the European Union regulation No 29/2012, geographic-origin labeling becomes a prerequisite. The authenticity of EVOO and VOO is of major importance for both financial and health-related reasons. To this end, various analytical techniques, able to detect possible adulteration and consequently ensure its quality and authenticity, have been developed. Among them, spectroscopic techniques such as Nuclear magnetic resonance (NMR), infrared (near, mid, far) and Raman spectroscopy and chromatographic methods such as liquid and gas chromatography have been used by various researchers to determine qualitative and quantitative changes in the composition of EVOO and VOO that are related to adulteration issues [7; 8; 9; 10]. Among the techniques mentioned above, gas chromatography is the most used technique regarding authentication issues that deal with geographical and botanical discrimination, while infrared and Raman spectroscopy are commonly used in cases of adulteration detection, as both are less time-consuming techniques and require no sample preparation. Due to the complex nature of the obtained data, manipulation with statistical methods so as to obtain a final report should be followed. In this regard, multivariate analysis has been proven to be a valuable tool. Many studies have pointed out the usefulness of this method and support its use in many sciences such as chemistry, health sciences and analytical chemistry [11; 12; 13; 14]. The goal of this review paper is to provide information over the last 20 years about olive oil authentication. Specifically, the review focuses on EVOO and VOO, the most consumed types of olive oil, and describes rapid techniques able to accurately predict their authentication. Articles regarding Infrared (IR) and Raman spectroscopy as well as gas chromatography (GC), able to detect adulteration and to classify EVOO and VOO according to their geographical and botanical origin, are presented by giving a brief summary of the vast available published information. Spectroscopic Techniques Used to Evaluate Adulteration of EVOO and VOO: Focus on Infrared and Raman Spectroscopy  
IR spectroscopy is a fundamental and popular method for the qualitative and quantitative analysis of complex matrices. The information generated is attributed to the presence of characteristic groups such as alcohols, ketones, alkenes, etc.