Age limits are effective in reducing alcohol- and tobacco-related harm, however, their effectiveness depends on the extent to which they are complied with. This study aimed to investigate the effectiveness of different age verification systems (AVSs) implemented by 400 Dutch supermarkets on requesting a valid age verification (ID) and on sellers' compliance. A mixed method design was used. Compliance was measured by 800 alcohol and tobacco purchase attempts by 17-year-old mystery shoppers. To analyze the effectiveness of AVSs, logistic regression analyses were performed. Insight into facilitating and hindering factors in the purchase process was obtained by 13 interviews with supermarket managers. Only a tendency toward a positive effect of the presence of the keying-on-date-of-birth AVS or ID swiper/checker was found on ID request for both alcohol and tobacco purchase attempts. The use of the keying-on-date-of-birth AVS or ID swiper/checker significantly increased the odds for compliance after an ID was requested, for both alcohol and tobacco purchase attempts. Managers indicated that ID requests and compliance could be facilitated by providing cashiers with sufficient managerial support, technical support, and regular training about the purchase process and use of the AVS. The usage of AVSs calculating and confirming whether the customer reached the legal purchase age for cashiers significantly increases the odds for cashiers to comply with age limits of alcohol and tobacco. Future research should gain insight into how usage of effective AVSs can be improved and explore the feasibility of implementation and effectiveness in other outlets. Copyright © 2016 The Society for Adolescent Health and Medicine. Published by Elsevier Inc. All rights reserved.