This book is not an introduction to Prolog. It is an explanation of how software systems written in Prolog are constructed and what they can do. The book would be of interest to those who are planning to write large programs of more than 500 lines, those specifically interested in programming assistance systems, editors and relational databases and those who are interested in studying practical examples of programming techniques using Prolog. The first part of the book is a comparison of several Prolog implementations. The following three sections are concluded with complete listing of the programs under discussion . These programs include a Prolog Programming Assistance System (PAS) which is designed to facilitate programming in Prolog, how an editor can be designed using Prolog and how to design a relational database using Prolog. The last section is an introduction to writing expert systems in Prolog and a discussion of the advantages and disadvantages of using Prolog when it is considered as a programming language or as an inference system.