This SAE Aerospace Information Report (AIR) outlines concepts for the design and use of fault isolation equipment that have general application. The specific focus is on fault isolation of environmental control systems (ECS) in commercial transports. Presented are general fault isolation purposes@ design principles@ and demonstration of compliance criteria. These are followed by three design examples to aid in understanding the design principles. Future trends in built-in-test-equipment (BITE) design are discussed@ some of which represent concepts already being implemented on new equipment. Purpose: This AIR provides a practical guide for the design and use of fault isolation systems for the ECS in commercial transports. Airframe manufacturers may use this AIR for guidance in developing practical@ efficient@ and workable ECS fault isolation equipment. Equipment suppliers may use this AIR for guidance concerning their role with airframe manufacturers in the development and with the operators in the use of effective ECS fault isolation equipment. Operators may use this document to write future ECS fault isolation specifications against which the performance of the delivered system can be measured quantitatively. They may also obtain guidance for making more effective use of existing systems.