The main problem targeted by this research is the lack of flexibility of virtual objects within a Virtual Environment. Virtual objects are difficult to re-use in applications or contexts different from the ones they were designed for. This refers mainly to the need for adaptive entities, from the points of view of the geometric representation and user interface. The solution we propose is based on a semantic model aimed at representing the meaning, and functionality of objects in a virtual scene; including their geometric representation and user interface. This thesis is divided into four main parts. First we define a semantic model for Virtual Environments populated by digital items (virtual entities). Then we focus on three main aspects of the development and use of Virtual Environments: interaction, visualization and animation. We describe the needs and benefits of using a semantics-based representation for: animating virtual characters; interacting within Virtual Environments and visualizing them in different contexts/devices (handhelds, hi-resolution displays, text-based devices, etc.). The thesis concludes by exploring a novel trend in research that builds upon knowledge-base technologies and the emerging Semantic Web initiatives: the use of ontologies for Virtual Environments. A preliminary version of an ontology for Virtual Humans is presented. When using ontologies, digital entities can be understood by both humans and machines and be reused in a wide range of contexts and applications. Semantic Virtual Environments are composed of such reusable and adaptive digital items.