Diverse businesses, such as garbage collection, retail banking, and management consulting are often tied together under the heading of “services”, based on little more than a perception that they are intangible and do not manufacture anything. Such definitions inadequately identify managerial and operational implications common among, and unique to, services. We present a “Unified Services Theory” (UST) to clearly delineate service processes from non-service processes and to identify key commonalities across seemingly disparate service businesses. The UST defines a service production process as one that relies on customer inputs; customers act as suppliers for all service processes. Non-services (such as make-to-stock manufacturing) rely on customer selection of outputs, payment for outputs, and occasional feedback, but production is not dependent upon inputs from individual customers. The UST reveals principles that are common to the wide range of services and provides a unifying foundation for various theories and models of service operations, such as the traditional “characteristics of services” and Customer Contact Theory. The UST has significant operational corollaries pertaining to capacity and demand management, service quality, services strategy, and so forth. The UST provides a common reference point to which services management researchers can anchor future theory-building and theory-testing research.