This review aimed to identify effective strategies to increase response rates to postal and electronic questionnaires. The review included 481 eligible trials for postal questionnaires and 32 eligible trials for electronic questionnaires. The findings showed that monetary incentives significantly increased response rates, with an odds ratio of 1.87 (95% CI 1.73 to 2.04). Other effective strategies included recorded delivery, a teaser on the envelope, a more interesting questionnaire topic, pre-notification, follow-up contact, unconditional incentives, shorter questionnaires, providing a second copy of the questionnaire at follow-up, mentioning an obligation to respond, and university sponsorship. Non-monetary incentives also increased response rates, with an odds ratio of 1.15 (95% CI 1.08 to 1.22). For electronic questionnaires, non-monetary incentives, shorter questionnaires, including a statement that others had responded, and a more interesting topic increased response rates. The use of a lottery with immediate notification of results, an offer of survey results, and using a white background also increased response rates. The odds of response were reduced when the email subject line included "Survey" or when the email included a male signature. The authors concluded that health researchers can increase response rates using the strategies shown to be effective in this systematic review.This review aimed to identify effective strategies to increase response rates to postal and electronic questionnaires. The review included 481 eligible trials for postal questionnaires and 32 eligible trials for electronic questionnaires. The findings showed that monetary incentives significantly increased response rates, with an odds ratio of 1.87 (95% CI 1.73 to 2.04). Other effective strategies included recorded delivery, a teaser on the envelope, a more interesting questionnaire topic, pre-notification, follow-up contact, unconditional incentives, shorter questionnaires, providing a second copy of the questionnaire at follow-up, mentioning an obligation to respond, and university sponsorship. Non-monetary incentives also increased response rates, with an odds ratio of 1.15 (95% CI 1.08 to 1.22). For electronic questionnaires, non-monetary incentives, shorter questionnaires, including a statement that others had responded, and a more interesting topic increased response rates. The use of a lottery with immediate notification of results, an offer of survey results, and using a white background also increased response rates. The odds of response were reduced when the email subject line included "Survey" or when the email included a male signature. The authors concluded that health researchers can increase response rates using the strategies shown to be effective in this systematic review.