The Grades of Recommendation, Assessment, Development and Evaluation Working Group (GRADE Working Group) has developed a system for grading the quality of evidence. Over 20 organizations including the World Health Organization (WHO) have adopted it. The quality of a body of evidence involves consideration of within-study risk of bias (methodological quality), directness of evidence, heterogeneity, precision of effect estimates and risk of publication bias, and the system considers the assessment of the quality of a body of evidence for each individual outcome. The GRADE approach specifies four levels of quality. Randomized trials, usually the highest grade of evidence, can be downgraded depending on the presence of the methodological problems. A brief overview of the grade system is presented.