This paper provides an overview of the methodologies and techniques currently being employed in geotechnical engineering and engineering geology fields and examines their relevance to waste disposal site design in arid and semi-arid environments. The methodologies covered are: semi-quantitative, qualitative and knowledge-based systems. Various fundamentals and limitations associated with each of the techniques are discussed. The combination of semi-quantitative and qualitative techniques in developing Knowledge-Based System Model Methodologies for evaluating the performance and design of waste disposal sites in arid and semiarid environments can provide relevant and sufficient data, and reduce uncertainty in the final results. However, such systems should be aimed at giving advice rather than attempting to replace human expertise.