Graphical User Interfaces used to be static, graphically representing one software state after the other. However, animated transitions between these static states are an integral part in modern user interfaces and processes for both their design and implementation remain a challenge for designers and developers. This paper proposes a Petri net model-based approach to support the design, implementation and validation of animated user interfaces by providing a complete and unambiguous description of the entire user interface including animations. A process for designing interactive systems focusing on animations is presented, along with a framework for the definition and implementation of animation in user interfaces. The framework proposes a two levels approach for defining a high-level view of an animation (focusing on animated objects, their properties to be animated and on the composition of animations) and a low-level one dealing with detailed aspects of animations such as timing and optimization. A case study (in the domain of interactive Television) elaborating the application of the presented process and framework exemplifies the contribution.