The NeOn Toolkit is the ontology engineering environment originally developed as part of the NeOn Project and now supported, together with other technologies from NeOn, by the NeOn Foundation. The NeOn toolkit is a state-of-the-art, open source multi-platform ontology engineering environment, which provides comprehensive support for the ontology engineering life-cycle. The toolkit is based on the Eclipse platform, a leading development environment, and provides an extensive set of plug-ins (currently 45 plug-ins are available) covering a variety of ontology engineering activities, including Annotation and Documentation, Development, Human-Ontology Interaction, Knowledge Acquisition, Management, Modularization and Customization, Neon Plugins, Old Main Page, Ontology Dynamics, Ontology Evaluation, Ontology Matching, Reasoning and Inference, Reuse.