Action Semantics is a framework for defining the semantics of languages. It is intended to be accessible to a wider audience of Computer Scientists than traditional semantics frameworks (such as Denotational Semantics). There has been little work carried out to date on the techniques required to define object-oriented languages with Action Semantics. The work presented in this thesis examines four potential approaches to defining the Action Semantics of object-oriented languages. In order to illustrate the four approaches a simple language EIL (Example Inheritance Language) is given, and described using these four approaches. The language Smalltalk-80 has been selected for a case study of a practical application of one of the techniques described above. It is important to be able to relate Action Semantics definitions of object-oriented languages to similar definitions given in other frameworks. It is described how this can be achieved. An example is given for the Action Semantics and Denotational Semantics of Smalltalk. This thesis concludes that it is feasible to produce Action Semantics definitions of object-oriented languages.