VLSI CAD applications deal with design objects that have an interface description and an implementation description versions of design objects have a common interface but differ in their implementation. versions of design object is a modeling construct which enables a database entity to be represented by two sets of heterogeneous records; one set describes the object's interface and the other describes its implementation. thus a reasonable starting point for modeling design objects is to begin with the concept of molecular objects. in this paper we identify modeling concepts that are fundamental to capturing the semantics of VLSI CAD design objects and versions in terms of molecular objects a provisional set of user operations on design objects, consistent with these modeling concept is also defined. the modeling framework that we present has been found useful for investigating physical storage techniques and change notification problems in version control.