The consequences of mastitis on the technological properties of milk and on the quality of milk products are widely reported in the literature. Besides, recent advances have shed light on the mechanisms involved in the udder response and subsequent milk changes in mastitis cases. This review gives an update on the literature regarding the impact of mastitis on milk composition and processing properties and collates recent data regarding the mechanisms involved in mastitis effects. It is an attempt to link field observations and experimental studies in order to better understand how mastites affect so dramatically the technological properties of milk. Both bovine and small ruminant milks are considered and a special emphasis is given on the role of staphylococci, streptococci, and Escherichia coli, the most common causative agents of mastitis.