Summary: As an integral part of a two-year professional development program, 85 elementary teachers developed and administered grade-level specific mathematics performance assessments in number sense and operation sense ‒ two assessments per grade level. Seven criteria based upon the NCTM Principles and Standards (2000) Learning Principle and Process Standards were created and then used to develop a rubric with a scoring hierarchy based upon the degree of assessor prompting required in order to ensure some degree of student success. Both assessments also included an instruction guide and suggested assessor questions or prompts to ensure that opportunities were provided for students to engage in performances that mirrored the criteria and addressed the components of number and operation sense as identified in the Principles and Standards. A ’Hierarchy of Numerical Complexity’ to guide the determination of the students’ level of numerical functioning relative to both tasks was then devised. A ’Place Value Hierarchy’ based upon the Ross Five-Stage Place Value Understanding Model (1990, 1999) was developed with the number sense assessment, in order to designate varying levels of place value comprehension. An ’Algorithm Guide’ based upon the Cognitively-Guided Instruction single digit and multidigit invented algorithms was also developed for labeling computational strategies in the operation sense assessment. Videotaped assessment administrations were utilized to teach assessment procedures and to enhance inter-rater reliability. Statistical analyses using Cronbach’s Alpha were used to establish that reliability among teachers.