Technology development in HCI can be interpreted as a co-evolution of tasks and artifacts. The tasks people actually engage in (successfully or problematically) and those they wish to engage in (or perhaps merely to imagine) define requirements for future technology, and specifically for new HCI artifacts. These artifacts, in turn, open up new possibilities for human tasks, new ways to do familiar things, entirely new kinds of things to do. In this paper we describe psychological design rationale as an approach to augmenting HCI technology development and to clarifying the sense in which HCI artifacts embody psychological theory. A psychological design rationale is an enumeration of the psychological claims embodied by an artifact for the situations in which it is used. As an example, we present our design work with the View Matcher, a Smalltalk programming environment for coordinating multiple views of an example application. In particular, we show how psychological design rationale was used to develop a view matcher for code reuse from prior design rationales for related programming tasks and environments. 1. TASKS AND ARTIFACTS In 1605, Sir Francis Bacon called for a "natural history of trades." He urged that technical tools, techniques and processes be made more public and explicit. This was one element in his broader project of developing practical science, and hinged on the assumption that if such knowledge could be more systematically considered and integrated, human progress would necessarily result. Thus, Bacon suggested that new concepts and inventions would result "by a connexion and transferring of the observations of one Arte, to the use of another, when the experiences of several misteries shall fall under the consideration of one man's minde."(1970: Book...