Foreign information manipulation and interference (FIMI) is not a new phenomenon. Since the end of the Second World War, when Sweden seriously began to work with psychological defence to respond to various forms of hybrid threats, the issue has been prevalent and developing. The report Psychological Defence: Concepts and Principles for the 2020s not only portrays the evolution of Sweden's psychological defence from the 1950s to its accentuated relevance today but also highlights how modern psychological defence differs from previous forms. While there are clear connections to the historical approach, essential differences, such as threats in the digital domain and the importance of increased international cooperation are emphasized. The authors elaborate on psychological defence according to four principles (resilience, threat intelligence, deterrence, and strategic communication). This report was written by the Psychological Defence Research Institute for the Swedish Psychological Defence Agency.