An expert system is a computer system which is capable of performing at the level of a human expert. The objective is to allow an unavailable human expert to be replaced with a computer. Under certain circumstances it has been possible to create expert system which perform better than human experts. This is not because the computer knows more than the human expert, in fact it knows considerably less, but because it applies what it does know in a relentlessly thorough manner.

The Agricultural Research Service of the USDA has developed the cotton Crop Management Expert System--COMAX, which advises cotton growers regarding the application of nitrogen, irrigation and growth regulators. COMAX, like most expert systems, consists of a knowledge base and an inference engine. The knowledge base is composed of a set of facts and a set of if-then rules. The third component to COMAX is a cotton plant model called GOSSYM. This model was developed by a team of agriculture researchers over a period of twelve years.