This thesis examines the process of designing effective transmedia narratives that are told across multiple media. It reviews existing transmedia projects and develops an ontology and framework for transmedia narrative design. The ontology defines the concepts and relationships involved in transmedia narratives, such as narrative elements, user engagement, and interaction design. It also presents a four-level design process covering the project, storyworld, story, and scene/sequence levels. The goal is to provide structure for transmedia design teams and help improve quality, efficiency and reduce costs of developing transmedia projects.