The language DLP introduced in the previous chapter is meant as a vehicle for the specification and implementation of distributed knowledge based systems. As an illustration of such an application, the specification and implementation of a distributed medical expert system will be described in section xps. Traditional problems encountered in software engineering, such as modularization and the distribution of control, reoccur when implementing knowledge based systems. Solving these problems becomes increasingly urgent when introducing parallelism in order to distribute the problem solving activity among distinct computing agents. In this chapter examples will be given that illustrate how DLP may be applied to object oriented modeling and distributed problem solving. An important contribution of the object oriented approach to programming has been the use of inheritance for defining the behavior of objects in an incremental way. As an example of the use of inheritance it will be shown how to factor out control in searching a state space. Factoring out control is also achieved by regarding the state space as an object that is given as a parameter to the search procedure. Further, an object oriented solution to the N-queens problem will be presented, where each queen is modeled as an active object busy satisfying the constraints imposed by the problem. Inheritance also provides a convenient mechanism for representing the knowledge contained by an object, in terms of its relation to other objects. It will be shown how inheritance is used to create objects that may be considered as a specialization of the inherited object. Further, examples will be given that illustrate how inheritance may be effected dynamically by delegating messages, and how inheritance may affect the backtracking behavior of an object. The specification and implementation of the distributed medical expert system illustrates that the object oriented paradigm provides a natural means of distributing the problem solving tasks following the hierarchical representation of the knowledge.