Abstract Objective To clarify the GRADE (grading of recommendations assessment, development and evaluation) definition of certainty of evidence and suggest possible approaches to rating certainty of the evidence for systematic reviews, health technology assessments and guidelines. Study Design and Setting This work was carried out by a project group within the GRADE Working Group, through brainstorming and iterative refinement of ideas, using input from workshops, presentations, and discussions at GRADE Working Group meetings to produce this document, which constitutes official GRADE guidance. Results Certainty of evidence is best considered as the certainty that a true effect lies on one side of a specified threshold, or within a chosen range. We define possible approaches for choosing threshold or range. For guidelines, what we call a fully contextualized approach requires simultaneously considering all critical outcomes and their relative value. Less contextualized approaches, more appropriate for systematic reviews and health technology assessments, include using specified ranges of magnitude of effect, e.g. ranges of what we might consider no effect, trivial, small, moderate, or large effects. Conclusion It is desirable for systematic review authors, guideline panelists, and health technology assessors to specify the threshold or ranges they are using when rating the certainty in evidence.