The first part of the thesis provides an introduction to the logic programming language Prolog and some areas of current research. The use of compilation to make Prolog faster and more efficient is studied and a modified representation for complex structures is presented. Two programming tools are also presented. The second part of the thesis focuses on one problem which arises when implementing an Expert System using Prolog. A practical three-valued Prolog implementation is described. An interpreter accepts three-valued formulae and converts these into a Prolog representation. Formulae are in clausal form which allows disjunctive conclusions to rules. True and false formulae are stated explicitly and therefore the interpreter is able to perform useful consistency checks when information is added to the data base.