The purpose of this article is to move the field of intellectual and closely related developmental disabilities (IDD) towards a better understanding of evidence and evidence-based practices. To that end, we discuss (a) different perspectives on and levels of evidence, (b) commonly used evidence-gathering strategies, (c) standards to evaluate evidence, (d) the distinction between internal and external validity, and (e) guidelines for establishing evidence-based practices. We also describe how the conceptualization and use of evidence and evidence-based practices are changing to accommodate recent trends in the field.