The Standards for Mathematical Practice (SMPs; CCSSI, 2010) describe mathematical behaviors and habits that students should express during mathematics instruction. Teachers should have knowledge about the SMPs, their meanings, and their implications for what students are expected to do when engaging in and with mathematics. In this chapter. we describe the process and development of the Standards for Mathematical Practice Knowledge Assessment (SMP-KA). The purpose of this chapter is to share validity evidence for interpretations of outcomes from the SMP-KA as a measure of teachers’ knowledge related to the SMPs. The development of the SMP-KA drew upon on Anderson and Krathwohl’s (2001) framework describing Bloom’s Revised Taxonomy. The Revised Taxonomy’s dual dimensions of knowledge and cognitive process guided construction of the instrument to ensure that important elements of the complexity and depth of the SMPs are assessed. Knowledge of the SMPs involves both mathematics content and a knowledge of students’ behaviors as they engage with mathematics. The SMPs are an important part of a mathematics teacher’s knowledge for teaching. The SMP-KA is an instrument to measure in-service K-12 teachers’ complex knowledge on the SMPs. The gathering of validity evidence and the instrument development process for the SMP-KA followed the standards of the American Educational Research Association, the American Psychological Association, and the National Council on Measurement in Education (American Educational Research Association, American Psychological Association, & National Council on Measurement in Education, 2014). The chapter provides sufficient validity evidence to support use of the SMP-KA to assess teacher knowledge of the SMPs.