This thesis aimed to evaluate the effects of the dietary inclusion of three by-products in sheep diets: Tomato pomace (TP) is the by-product derived from process of tomato juice, grape marc (GM) derived from winemaking and exhausted myrtle berries (EMB) derived from production of liqueur called Mirto rosso (red myrtle).This study is divided in 4 main chapters.Chapter 1 reports the literature on amounts, chemical composition and fatty acid profile of by-products from tomato, wine and myrtle industries. Their use in ruminant nutrition and the effects on milk yield and composition and the biological effects have been discussed. Furthermore, the bioactive compounds, as polyphenol, identified in these by-products have been reported.In the others chapters of the thesis are reported: the study of the effects of use of small amounts of dried tomato pomace, grape pomace and exhausted myrtle berries on milk yield and composition, and blood biochemical parameters (Chapter 2); the additional information on nutrient and polyphenol composition of the by-products from tomato, wine and myrtle industries in order to supplement existing feedstuff tables (Chapter 2); the effects of tomato pomace, grape pomace and exhausted myrtle berries on oxidative status of animals and milk (Chapter 3); the effects of these by-products on milk fatty acid profile (Chapter 3); and, finally, the effects of by-products on rumen microbiome and fermentation parameters (Chapter 4).