Expert systems, a branch of artificial-intelligence studies, is introduced with a view to its relevance in transportation engineering. Knowledge engineering, the process of building expert systems or transferring knowledge from human experts to computers, is described. The general differences between expert systems and conventional computer programs are summarized. The architecture of the expert system is shown to separate knowledge of the problem domain (knowledge base) from general problem solving knowledge (inference engine). Different approaches to each of these tasks are described. Recent developments in computer software that support and simplify the development of expert systems are presented, and recent applications to three transportation engineering problems are described. Background information is given to recommend the development of a prototype expert system for traffic control in construction zones.