The Accreditation Council for Graduate Medical Education (ACGME) requires that emergency medicine (EM) residency graduates are competent in the medical knowledge (MK) core competency. EM educators use a number of tools to measure a resident's progress toward this goal; it is not always clear whether these tools provide a valid assessment. A workshop was convened during the 2012 Academic Emergency Medicine consensus conference “Education Research in Emergency Medicine: Opportunities, Challenges, and Strategies for Success” where assessment for each core competency was discussed in detail. This article provides a description of the validity evidence behind current MK assessment tools used in EM and other specialties. Tools in widespread use are discussed, as well as emerging methods that may form valid assessments in the future. Finally, an agenda for future research is proposed to help address gaps in the current understanding of MK assessment.