Mastitis is an inflammation of the mammary glands primarily resulting from invasion by pathogenic microorganisms through the teat canal. The disease is caused by various microorganisms including bacteria, viruses, yeasts, moulds, algae, and some other fungal species. But, the most prevalent causes of mastitis are some gram negative and gram positive bacteria. Harari region had 8,200 cows that produce 3.1 million liters of milk per year and the town had more than 40 dairy farms consisting of entirely of cross breed cows that played a significant role in milk production for people of the town. This book,therefore,shows the overall prevalence of bacterial mastitis and its influencing risk factors in dairy cows of Harar town and the possible health risks posed by the bacterial species.