The trend in Environmental Control System (ECS) design for military aircraft is towards the use of closed loop cooling systems with the use of microprocessors for ECS control. The use of a closed loop ECS can reduce energy extraction from the engines thereby resulting in a reduction in aircraft takeoff gross weight penalties and/or an increase in aircraft range. The other advantages over a conventional open loop system are that the vulnerability to the Chemical Biological (CB) threat is reduced and a more favorable thermal environment is provided for avionics. The purpose of this study was to develop a digital control system for a closed loop ECS and to demonstrate the controls in a breadboard laboratory test rig. The study showed that a practical approach to systems development includes use of the “EASY digital computer program for control law development, a hybrid analog/digital computer for software development and demonstration in a breadboard ECS.