An object-oriented design is presented for building dynamic three-dimensional applications. This design takes the form of the Fifth Dimension Toolkit consisting of a set of interrelated classes whose instances may be connected together in a variety of ways to form different applications. Animation is obtained by connecting graphical objects to dynamic variables, which are able to change their values over time by responding to events. The Fifth Dimension Toolkit is the core of the Fifth Dimension Project, a research project for animating synthetic actors in their environment. The design philosophy and methodology of the toolkit are also described, as well as some of the implementation issues for the Silicon Graphics Iris 4D workstation