Strategic environment of the Republic of Serbia is determined by the operation of a large number of factors, which affect the defense system and its development in various ways. In the area of development planning of the defense system, the overriding purpose of analysis and assessment of the strategic environment is review and cogitation of phenomena and trends, and their influence on the development of the defense system. This paper analyzes the basic phenomena and trends in the field of strategic environmental factors and their individual, as well as hybrid, impact on the development of the defense system. For the sake of predictable and successful implementation of the defense system purpose, defense planning holders pay special attention to the analysis and assessment of the strategic environmental factors such as security, political, economic, social, technological, informational, ecological, and others. Strategic environmental factors are interdependent and connected, and they make an inseparable whole - the reality that affects functioning and development of the defense system. Knowledge and experience of defense analysts and planners is important for the quality assessment of the strategic environment impact and in the end it is crucial for proposing management decisions in order to ensure the effective functioning and development of the defense system. In the process, the application of scientific and field-proven methods, tools and techniques is inevitable. Study of the strategic environment impact creates the conditions for continuous and preventive action for timely adjustment of the defense system to new circumstances, thus enabling the successful execution of the defense tasks and protection of the defense interests.