The article explores basic personality characteristics in accordance with the Big Five inventory; it concerns the age category of adolescence, i.e. the ages 11–19. Particular attention is paid to the variables of age, sex, and nationality as factors contributing to the establishment of these personality characteristics. The research involved 2,830 adolescents aged 11–19. The average age was 14.89 (SD = 2.23). Participants attended lower or higher secondary education (ISCED 2 and 3) in the Czech Republic or the Slovak Republic. The research employed the Big Five Inventory BFI-10. While...