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In recent years, much work has been done on realistic animation. It is now possible to simulate and animate a variety of different types of objects and physical effects. However, little attention has been paid to the problem of interactively creating and editing realistic animated scenes. Developing and modifying such scenes is still a batch-oriented process that depends on trial and error. Animation editing for non-realistic scenes has also undergone rapid development. Presentation graphics systems, keyframe-based editors, and virtual reality-based interactive tools for a variety of purposes have been designed. However, physical realism is missing from these systems. The principal result of this dissertation is the design and implementation of the Asgard animation system, an editor for creating physically-based 3D animation. Asgard provides the user with a flexible multiple-representation editing interface, which allows both direct-manipulation graphical definition and modification of the scene, and textual editing of a formal description of the objects, forces, and other parameters of the animation. Multiple instances of each type of view can be created, all of which are synchronized. Such an interface allows the user to utilize the most convenient and effective tools for each task in the definition of the scene. Asgard also includes a differential equation solution technique that is adapted to the problem of physically-based animation, which uses an event-driven scheduling algorithm to take maximum advantage of the differing time-varying properties of different components in the scene while at the same time minimizing the penalties involved with collision detection and response. This algorithm partitions the state equation graph of the scene into strongly-connected components, which also allows Asgard to preserve as much previously-computed data as possible when editing changes are made by the user. Asgard is part of the multi-media document processing system Ensemble. Animated scenes can be integrated with multi-media documents, as well as developed on their own.