Assessment models support the design of quality performance assessments. Assessment tools are being developed to enable easy and effective application of the models. Based on representations of assessment design knowledge and domain knowledge in ontologies, the tools provide guidance to assessment designers, and through constraint processing check the completeness and accuracy of designs. With the addition of Bayesian networks, the tools can also enable individualized instruction by identifying knowledge gaps and prescribing instruction to fill the gaps. This paper describes the technical approach to developing the tools and discusses applications of ontologies and Bayesian networks for assessment authoring and individualized instruction.