A reprint of the article "Variety, Constraint, and the Law of Requisite Variety," by W. Ross Ashby, which appeared in the journal "Modern Systems Research for the Behavioral Scientist" is presented. The article discusses a role of the concept of a set plays in a communication theory developed by Claude Shannon and Wiener. It also explores the need for a set of messages by communication. The importance of constraint as a concept of communication is explained.