Background: Adequate knowledge is essential for making informed decisions. We attempted to determine the level of knowledge about mechanisms of action of birth control methods in five representative samples of European women., Study Design: Randomly selected women, aged 18-49 years, completed an anonymous survey in Germany, France, the UK, Sweden and Romania (N=1137). Participants were asked about how contraceptive methods work and if providers should inform them about this issue. Multiple linear regression was used to evaluate women's characteristics associated with their knowledge of mechanisms of action., Results: The majority of women identified the unequivocal mode of action of condoms, sterilization and abortion. Fewer than 2% identified all possible mechanisms of action of hormonal contraceptives and intrauterine devices. Highly educated women correctly identified the mechanism or mechanisms of action of more methods than less educated women (β=0.22, 95% confidence interval 0.01-0.43). Regardless of their sociodemographic characteristics and their belief about when human life begins, most women (75%) stated that the provider should inform them about possible postfertilization effects., Conclusions: European women have low knowledge about mechanisms of action of several contraceptive methods. The majority want to be informed about possible postfertilization effects. Since adequate knowledge is essential for making informed decisions, providers are encouraged to inform women about all possible mechanisms of action of contraceptives., (Copyright © 2012 Elsevier Inc. All rights reserved.)