Knowledge utilization is a field crossing many sectors, from agriculture, since the 1920s, to health care today. Evaluators have made long-standing contributions to understanding knowledge utilization. Different models or ways to think about knowledge utilization have evolved to reflect different perspectives, contexts, and stages of the process, from knowledge creation to the use of effectiveness results in policymaking. The rich interdisciplinary history of this field challenges evaluators to interrogate what knowledge (really) means within a policy or program--whether knowledge is being used more symbolically, rhetorically, or tactically, for example. Differences in program or policy effectiveness across different program sites might result from different types of knowledge use in those sites.