This thesis investigates the issues involved in the creation of a “general theory of operational semantics ” in LEGO, a type-theoretic theorem proving environment implementing a constructionist logic. Such a general theory permits the ability to manipulate and reason about operational semantics both individually and as a class. The motivation for this lies in the studies of semantics directed compiler generation in which a set of generic semantics transforming functions can help convert arbitrary semantic definitions to abstract machines. Such transformations require correctness theorems that quantify over the class of operational semantics. In implementation terms this indicates the need to ensure both the class of operational semantics and the means of inferring results thereon remain at the theorem prover level. The endeavour of this thesis can be seen as assessing both the requirements that general theories of semantics impose on proof assistants and the efficacy of proof assistants in modelling such theories. Acknowledgements First and foremost I would like to thank Kevin Mitchell who supervised me for my first four years, supplying me with many helpful hints and constructive criticisms. He also bore with me at a period of my life when my mental health deteriorated for which I am eternally grateful. Secondly I would like to thank Stuart Anderson an ever present of my life at the University since I first arrived in 1988, for taking over the supervision of my work when it was seemingly near its conclusion. The help and encouragement I received meant I was able to (finally!) complete this thesis. Special mention must go to Rod Burstall, my mentor through the entirety of my postgraduate studies. My all too brief encounters with him lifted my spirits at a time when they were desperately in need of a boost. I would also like to especially thank Thomas Kleymann (formerly Schreiber) for the many times he aided me in my Lego miseries. I also thank James Hugh McKinna, Randy Pollack and other members of the Lego club for their helpful ideas, various helpful officemates