An expert system is a computer system for inferring knowledge from a knowledge base, typically by using a set of inference rules. When the concept of expert systems was introduced at Stanford University in the early 1970s, the knowledge base was an unstructured set of facts. Today the knowledge base of expert systems is often given in terms of an ontology, extracted and built from various data sources by employing natural language-processing and statistics. To emphasize such capabilities, the term “expert” is now often replaced by “cognitive,” “knowledge,” “knowledge-based,” or “intelligent” system. With very few exceptions, general-purpose expert systems have failed to emerge so far. However, expert systems are applied in specialized domains, particularly in healthcare. The increasing availability of large quantities of data to organizations today provides a valuable opportunity for the application of expert systems, but also raises issues regarding privacy and legal liability.