Spoilage microorganisms cause changes of primary characteristics and properties of milk and dairy products. The product defects depends on the specific species and number of microorganisms involved in pre- and post- technological processing. Most often, these changes are related to single undesirable sensory characteristic, smell, flavour or conistency. However, in the case of heavier microbial contamination all these undesirable characteristics can occur simultaneously. Besides, even small changes caused by presence of spoilage microorganisms lead to decreased quality of milk and various dairy products. Despite of the importance for the overall quality, the control of spoilage microorganisms for dairy industry is not obligated and therefore, only a few producers control them. Therefore, the present study describes the undesirable effect of spoilage microorganisms on quality of raw, pasteurized and sterilized milk, fermented milk, butter, sour cream and cheeses with the intention to emphasize the importance and significance of their control in the dairy industry.