From 2013 to 2017, Rockman et al. developed and tested various strategies for assessing the pedagogical content knowledge (PCK) of novice and experienced STEM teachers. The research, conducted as part of Rockman's external evaluation of the "Mathematics and Science Teacher Education Residency" (MASTER) project, began with a review of existing tools, followed by a pilot study with metrics built around authentic student work. The metrics, customized for math and science teachers, asked teachers to analyze students' learning needs and offer ways to address them. While the research revealed differences within and across disciplines--in, for example, how teachers prioritize procedural and content knowledge--it also confirmed that PCK was a multifaceted, often elusory concept, variously defined by discipline, subject, classroom context, and topic. As researchers explored ways to create reliable tools that captured the nuances of PCK but still generated meaningful data that could inform MASTER development, partners continued to look for ways to fold PCK into teacher training in meaningful ways. Their development of specific objectives for what teachers with strong PCK do in the classroom allowed researchers to examine PCK "situated in practice," using not a single tool but multiple methods. Analyses of lesson plans over residents' clinical year and retrospective interviews with MASTER-trained teachers, in their induction year, provided complementary opportunities to explore instantiated PCK, defined by both classroom practice and teacher readiness. The lessons learned confirm the challenges of assessing PCK within and between subject areas, as well as some opportunities for research and development around teacher training. [For the complete volume, "Pedagogical Content Knowledge in STEM: Research to Practice. Advances in STEM Education," see ED613708.]