This paper discusses the use of ontologies and their applications to assessment. An ontology provides a shared and common understanding of a domain that can be communicated among people and computational systems. The ontology captures one or more experts' conceptual representation of a domain expressed in terms of concepts and the relationships among the concepts. The ontology standardized the terms and structures of the domain. The paper presents two examples of the use of ontologies for assessment purposes. The first example describes than application for the domain of rifle marksmanship, and the second example discusses the use of ontologies in the design of authoring systems to link assessment, content, and cognitive demands. The overall purpose for developing the ontology of rifle marksmanship was to capture the knowledge and structure of the domain in a way that allows the exploration of the use of ontologies for assessment and instructional purposes. In the other example, the ontology was developed to use as a domain template against which to score knowledge maps. The technology behind ontologies has matured to the point that computational tools are readily available. An ontology-based authoring system can impose structure on the authoring task and check that user-specified values simultaneously satisfy all constraints among the assessment components. (Contains 28 references.) (SLD)