The pioneering object-oriented programming system developed in 1972 by the Software Concepts Group, led by Alan Kay, at Xerox PARC between 1971 and 1983. It includes a language, a programming environment, and an extensive object library.
object message: param1 secondArg: param2 .. nthArg: paramN
Five pseudo-variables are defined: "self", "super", "nil", "true", "false". "self" is the receiver of the current message. "super" is used to delegate processing of a message to the superclass of the receiver. "nil" is a reference to "nothing" (an instance of UndefinedObject). All variables initially contain a reference to nil. "true" and "false" are Booleans.
In Smalltalk, any message can be sent to any object. The recipient object itself decides (based on the message name, also called the "message selector") how to respond to the message. Because of that, the multiple inheritance system included in the early versions of Smalltalk-80 appeared to be unused in practice. All modern implementations have single inheritance, so each class can have at most one superclass.
Early versions were Smalltalk-72, Smalltalk-74, Smalltalk-76 (inheritance taken from Simula, and concurrency), and Smalltalk-78, Smalltalk-80. Other versions include Little Smalltalk, Smalltalk/V, Kamin's interpreters. Current versions are VisualWorks, Squeak, VisualAge, Dolphin Smalltalk, Object Studio, GNU Smalltalk.