This paper presents basic principles of a scenario approach application to the problem of optimal development of social economic systems. The formalized description of scenario of system sustainable development is introduced and synergetic and attractive scenarios are examined. The modes providing stable and safe development of social economic system are investigated for synergistic scenarios, however for attractive scenarios it is the control purpose. Scenario design example based on the use of the social economic system development model represented by signed the graph language are given. Formal methods of such scenarios development, the "Object-subject" method and the method of base scenarios, are proposed.