Abstract Minimizing the cost of drilling a directional well is a major concern for the drilling engineer. This computerized system was developed in order to provide the drilling engineer with an advisory tool which recommends changes in the Bottom Hole Assemblies (BHA). This results in better accuracy and faster decision making; hence, considerable savings in time and money. The system involves five integrated modules: – A knowledge base where all the expertise concerning the subject is stored. – A database with information from the BHAs used in drilling of previous directional wells and their statistical behavior. – A database with information from the theoretical behavior of BHAs commonly used in directional drilling. – A database with information from directional surveys performed over the path of the bore hole. – Computerized applications developed to calculate and plot the path of the bore hole. This system was developed using an expert systems technology, including a knowledge base and a inference engine. The computerized applications for plotting and calculating the course were developed in a conventional way using BASIC and PASCAL: Databases were all developed in a relational model.