In this chapter several aspects of udder health are discussed. Mastitis, inflammation of the mammary gland, which is generally caused by bacterial infections, is one of the most important and most studied diseases in dairy cattle. Diagnostic approaches are discussed with specific attention for the bacteriological causes of the disease. Subsequently immunological aspects of intramammary infections will be reviewed. Because treatment of mastitis in unavoidable at some point in time in most dairy herds, attention is given to treatment of mastitis with an emphasis on different types of antibiotics and antibiotic resistance. The most important part of udder health management, however, is the preventive management. From that perspective, breeding, housing and nutrition are shortly discussed, as are the milking machine and milking procedures. Finally attention is given to problem solving once mastitis has led to a herd level problem and some future trends are discussed.