States the era of interoperable direct digital control (DDC) systems has begun, and the future holds the promise of more full-featured and competitively-priced offerings. Standards are being set, new products are in development and the industry is learning the advantages and disadvantages of this new technology. Consulting and specifying engineers face the challenge of determining what an owner needs from an interoperable system and then writing a competitive specification to achieve those needs. Considers why interoperability should be specified. Deals with difficulties in specifying and the use of prescriptive and performance-based specifications. Considers addressing interoperability, the functions of interoperable systems, and gives examples of interoperable system specification. In separate sections discusses methods for achieving interoperable systems and the five basic functions that the DDC system should be expected to perform regardless of the method selected to achieve interoperability.