Publisher Summary This chapter highlights systems science and its disciplines. Systems science is a new scientific discipline where the subject is systems, and the methodology is the systems approach to problems. These systems are sets of interrelated elements that may be defined with reference to natural or social subjects may be physical or informational, concrete or abstract, existing or planned, static or dynamic. The methods of systems science are used to define systems, to distinguish them from the environment, represent them, analyze and optimize their structure and behavior. The classification of systems science disciplines include (1) general systems theory, (2) cybernetics, (3) systems engineering, (4) operations research, and (5) systems analysis. The first and second fields may be classified as parts of the systems theory, the others as applications. The general systems theory deals with the most general scientific principles of the existence, description, and behavior of systems. Philosophical aspects are integral elements of the general systems theory. There are two categories of systems distinguished by their different gnostic essences—namely, (1) real (material) and (2) conceptual (ideal) systems.