Conjugated Linoleic Acid (CLA) are a group of positional and geometric isomers of Linoleic Acid characterized by a carbon chain containing 18 carbon atoms and two double bonds, not in the classic position (cis), but conjugated from the carbons atoms 9, 10 or 11. Double bonds have different position in the carbon chain ([7,9], [8,10], [9,11], [10,12], [11,13] and [12,14]) and four different geometric distribution (cis/trans, trans/cis, cis/cis and trans/trans). In total 24 possible isomers are identify, but the two most present and often most identified are: C18:2cis9,trans11 (60-85% of the isomers identify in meat and >90% in milk) and C18:2trans10,cis12 isomers. Observing meat samples other isomers are commonly identified, as example, C18:2trans7,cis9 and C18:2trans11,cis13. In 1987, CLA have been discovered thanks to the research group of Dr. Pariza that studying some carcinogenic components in grilled meat, identify these molecules with anticancer activity, called Conjugated Linoleic Acid. After the discovery, they still receive many attention because of their biological activities and implication on human health. The biological effect are due to the separate actions of the main isomers (C18:2cis9,trans11 and C18:2trans10,cis12) and sometimes by the synergistic action of both. Also, minor isomers have biological effect, as examples, C18:2trans9,trans11 that inhibits platelet aggregation and has anti-proliferative effect and C18:2cis9,cis11 that showed anticancer effect. In animal model, CLA not only reduce initiation, promotion and progression steps of cancer development, but also reduce metastasis. Nevertheless, was show that CLA isomers are important because of they have effect on animal performance and the principal effects are: prevent chemically-induced tumors, protect against the catabolic effects of immune stimulation, improve feed efficiency, reduce excess body weight gain, reduce body fat, increase lean body mass and lower blood lipids. In human diet, the main sources of CLA are representing by ruminants products, in particular milk, dairy product and beef. The higher concentration of CLA in these products is linked to the presence of rumen that through microbial biohydrogenation (Butyrivibrio fibrisolvens) can transform Linoleic Acid to the major isomer C18:2cis9,trans11 (Rumenic Acid). Conjugated Linoleic Acid (CLA) can be analyze with different methods of lipid extraction and derivatization, but always after transformation in methyl ester derivatives of fatty acids (FAMEs) that is carried out in a simple reaction. This simple reaction becomes more complicated because of conjugated fatty acids are involved and the presence of conjugated double bond makes them unsuitable for the most common techniques employed for fatty acids analysis. The presence of double bonds may increase isomerization and epimerization of these bonds, increasing CLA with trans/trans configuration and reducing cis/trans or trans/cis configuration. For this reason is very important find a suitable method for lipid extraction (and the resulting determination of crude fat) and subsequently for the transformation in methyl ester derivatives of fatty acids (FAMEs). For this reason at the first part of my thesis was given a methodological approach in order to understand the differences between methods and which is the most suitable (Chapter 2 and Chapter 7). All the data used, belonging to a trial carried out at “Lucio Toniolo”, the Experimental Farm of the University of Padova in Legnaro (Padova, Italy). Animals used are a crossbreed between Belgian Blue bulls and Brown Swiss dairy cows fed with one of 3 experimental diets. Diets were composed by unifeed and differ depending on the rumen protected CLA supplementation (rpCLA), overall, three supplementation are available 0, 8 and 80 g/d/animal. The trial ended in March 2011 when animals were slaughtered outside the faculty in a slaughterhouse located in Pergine province of Trento (Trentino Alto Adige Region). Samples were collected and in particular three tissues were subjected to analysis for fatty acids profile and CLA content: muscle Longissimus Thoracis, Subcutaneous Fat and Liver. In the main Chapter (Chapter 2) tissues (Longissimus Thoracis, Subcutaneous Fat and Liver) were analyzed for determine fatty acids profile comparing three different methods of extraction (Folch (1957), Accelerated Solvent Extraction (ASE) and Jenkins (2010)) using as chromatography technique two dimensional GC (GC×GC). The purpose of this trial was identify among the different methods of extraction, one method that is able to identify CLA isomers without causing isomerization. Folch (1957) is one of the older and most used methods. It was born for analyze samples rich in phospholipids (as lipids of brain). It works at room temperature using a mixture of solvents composed by chloroform/methanol (2:1, v/v). Accelerated Solvent Extraction (ASE) was used with the purpose to compare a room temperature method with a method that works at high temperature and pressure (120°C and 20MPa) with the same mixture of solvents (chloroform/methanol, 2:1, v/v). This method, which have recently been introduced; reduce the use of solvent and saving time at work, giving results that were similar or better if compared with the conventional Folch (1957) extraction. On the contrary, it is expected to increase the isomerization of double bonds and the isomers with trans/trans configuration. The last method, Jenkins (2010), is a direct method, which reduces the length of the total procedure, saving time at work, reducing the sample amount, reducing the use of solvents, giving analysis less expensive and easier. The main characteristic is that lipid extraction step is avoided and fatty acids are extracted and trans-esterified in the same time. The statistical analysis was performed in four steps: resolution power and assessment of the number of undetected FA, study the main sources of variation, using Levene’s test explored the variances homogeneity for the main sources of variations and relationships between methods. The incidence of undetectable values on the total number of expected observation, which depend on the sensitivity of the method used, for liver, fat and muscle, ranged 0.04 to 0.08, 0.05 to 0.06, and 0.05 to 0.12, respectively, with incidences greater for the Jenkins method compared to the other two for liver and muscle samples but not for subcutaneous fat. In liver the highest incidence of null values with the Jenkins method was mainly observed for short chain FA (C8:0 and C10:0), whereas in the case of muscle the highest incidence of undetectable values was mainly observed for the C24:0, for two C20:1t unknown isomers, and for Ω3 (C20:3n3, C20:4n3, C22:5n6). Many sources of variation result high significant (P