Software of today becomes more and more large, and it provides more and more services. Usually, the“selection-execution”model is used to invoke available services of a software. Therefore, there may be many services in a software which have never been invoked by its users, or whose existence is unknown to its users. This paper introduces a notion called active services. An active service is a service which can invoke itself under some circumstances, although its users may not know its existence. It also presents three basic models of active services and demonstrates their applications in various software.