Classical and modern concepts of system analysis are under consideration in the article. Main complex systems attributes are analyzed with a special attention on the attributes possessed by the most complex of all systems – biological systems. The reasons of why the technical science representatives and some other scientists have mechanical view on the system analysis are grounded. The essence of system response as a main system attribute is considered. The role of system conjugation is analyzed. The article shows the role of energy in the system activity. The necessity of system analysis for environmental safety assessment is explained in the article. The author presents the definition of terms such as system and system analysis.