Work on item response theory was extended to two areas not extensively researched previously, including models for: (1) test items that require more than one ability for a correct response (MIRT); and (2) interaction between modules of instruction that have a hierarchical relationship (HST). In order to develop the MIRT and HST models, the author evaluated characteristics of the models, developed estimation procedures for the parameter of the models, and evaluated the models on their ability to describe real test data. A summary of the research is presented here, and references are made to papers and technical reports containing more detailed descriptions of the research efforts. (Author/LMO)