volution, both with respect to increase in size and to the introduction of new services, the degree of decentralized control tends to vary considerably over time. The abstraction hierarchy of the service network is, however, reasonably stable over time. It is therefore important that the service network model distinguishes between user-provider relations and relations between distributed agents. Service resource models are the fundamental specification elements in service network modelling, not only because they define the abstraction hierarchy of the network, but also because they are the basis for behaviour specification. The first step in specifying behaviour is to assign a set of operations to each service resource model. Each operation defines a set of ASPs. When service objects communicate, they use communication events, which are instances of ASPs. The second step in specifying the behaviour of a service object is to create an object resource model for the object, by integrating all service resource models that concern the service object. This integration is achieved by defining abstrac- tion relations between service resource models on different abstraction levels, and external resource relations between service resource models on the same abstraction level. Since abstraction and external resource relations define map- ping between ports of the service object, they are also the prerequisites for describing object behaviour. From a behavioural point of view, most service objects can be regarded as state machines. A state machine performs transitions between states. No symbols other than states and transition symbols are allowed in AML state diagrams. A behaviour description can be based on reactive states or active states, or a mix of both types of states. When an object is in a reactive state, it is prepared to take 6 Process Control System Modelling inputs from other objects to which it has service relations. When an object is in an active state, it is not normally prepared to take an input. However, an 