This review assesses the effects of decision aids on people facing health treatment or screening decisions. Decision aids are interventions that support patients by making their decisions explicit, providing information about options and associated benefits/harms, and helping clarify congruence between decisions and personal values. The review includes 105 studies involving 31,043 participants from 10 countries. The primary outcomes evaluated included attributes of the choice made (knowledge, accurate risk perceptions, and congruence between informed values and care choices) and attributes related to the decision-making process (decisional conflict, patient-clinician communication, participation in decision making, and proportion undecided). Decision aids were found to increase participants' knowledge, accuracy of risk perceptions, and congruency between informed values and care choices compared to usual care. They also reduced decisional conflict, improved patient-clinician communication, and increased satisfaction with the decision and decision-making process. Decision aids did not worsen health outcomes or reduce satisfaction. The review found no adverse effects on health outcomes or satisfaction. New evidence indicates that decision aids improve knowledge and accurate risk perceptions when used either within or in preparation for the consultation. Further research is needed on adherence with chosen options, cost-effectiveness, and use with lower literacy populations.This review assesses the effects of decision aids on people facing health treatment or screening decisions. Decision aids are interventions that support patients by making their decisions explicit, providing information about options and associated benefits/harms, and helping clarify congruence between decisions and personal values. The review includes 105 studies involving 31,043 participants from 10 countries. The primary outcomes evaluated included attributes of the choice made (knowledge, accurate risk perceptions, and congruence between informed values and care choices) and attributes related to the decision-making process (decisional conflict, patient-clinician communication, participation in decision making, and proportion undecided). Decision aids were found to increase participants' knowledge, accuracy of risk perceptions, and congruency between informed values and care choices compared to usual care. They also reduced decisional conflict, improved patient-clinician communication, and increased satisfaction with the decision and decision-making process. Decision aids did not worsen health outcomes or reduce satisfaction. The review found no adverse effects on health outcomes or satisfaction. New evidence indicates that decision aids improve knowledge and accurate risk perceptions when used either within or in preparation for the consultation. Further research is needed on adherence with chosen options, cost-effectiveness, and use with lower literacy populations.