Module 2 of the course on Food Hygiene and Industrial Microbiology aims at understanding the effect of the presence and development of alterative and virtuous microorganisms in food. In particular, the knowledge concerning the main alterations originated by micro-organisms in food and to the changes made by micro-organisms in fermented foods must be developed. The intrinsic, extrinsic and implicit factors that influence the development and survival of microorganisms in food must also be known. Knowledge of these aspects will provide the essential elements for understanding the role of micro-organisms in food production and their impact on food quality. The use of this knowledge will also allow us to understand the principles of micro-organism control through physical, chemical and biological treatments or combinations thereof. The teaching aims to put the student in a position to draw conclusions about the effect of the presence and development of different microorganisms in food.