California's Commission on Teacher Credentialing (Commission) requires all programs of preliminary multiple and single subject teacher preparation to use a Commission-approved Teaching Performance Assessment (TPA) as one of the program completion requirements for prospective teacher candidates. Three TPA models were approved by the Commission: (1) the FAST (Fresno Assessment of Student Teachers), owned and operated by California State University, Fresno (Fresno State); (2) the edTPA, owned by Stanford University, with an operational contractor of the Evaluation Systems group of Pearson; and (3) the CalTPA (California Teaching Performance Assessment), originally developed by Educational Testing Service (ETS) and owned by the Commission, revised by a Design Team with an operational contractor, also of the Evaluation Systems group of Pearson. Although each TPA model is Commission-approved, they differ in several important ways--for example, in the design of candidate tasks or cycles and scoring rubrics. Further, each TPA model must address all Teaching Performance Expectation (TPE) domains but need not address all elements. These inter-model differences raise questions regarding the comparability of results obtained by teacher candidates completing the various TPAs. Consequently, the Commission contracted with the Human Resources Research Organization (HumRRO) to conduct an independent investigation of the comparability of the three TPA models. The investigation occurred between June 2017 and December 2019. The technical approach to the evaluation marshalled evidence from numerous sources such as stakeholder surveys, analysis of score patterns, and comparisons to a common criterion. The goal was to accumulate as much evidence as possible to evaluate the comparability of the three TPA models. [For Volume II: Appendices, see ED613835.]