The aim of the project is to evaluate the breeding systems of fattened pigs in the Czech Republic in terms of the level of long-term animal stress, which is the trigger of aggressive behavior and the cause of tail biting, and further to define the main factors and predictors of the aggression occurrence in fattening pigs in relation to genotype, sex and heredity, based on the obtained data, then an assessment of the suitability of systems, technologies and possible enrichment of environment according to the ability to prevent aggression and injuries to animals. The goal is also to design and present the improvement of the living conditions of fattened pigs and thus the optimization of the production systems from a welfare point of view.