This monograph proposes how to manage complexity by organizing the system as a State Tree Structure (STS). Based on STS, which is an adaptation of statecharts to Supervisory Control Theory, an efficient recursive symbolic algorithm is presented that can perform nonblocking supervisory control design in reasonable time and memory for complex systems. Nonblocking Supervisory Control of State Tree Structures presents how this results in tractable and highly comprehensible controllers, especially to users who are not specialists in Discrete - Event Systems.