Abstract This paper presents a model of the engineering design process as sequences of transformations between states of the information models describing the design, the functional requirements on which it is based, and the constraints in which it is defined. The design itself is considered to be described by explicit attributes which are the definitive or descriptive properties which affect the manufacture of the object, and implicit, properties which are judgements, performance-related assessments, or emergent properties. The paper first considers the required characteristics of a design model and reviews the use of transformation in design. It then presents an overall transformation model followed by an overview of the use of Petri nets for modelling the detail of the processes.