The growing importance of researching online activities, such as cyber-deviance and cyber-crime, as well as the use of online tools (e.g. questionnaires, games, and other interactive tools) has created new ethical and legal challenges for researchers, which can be even more complicated when researching adolescents. In this article, we highlight the risks emerging from the current European legal and ethical landscape when researching potentially vulnerable groups, with a special focus on online research. It is not always clear how to differentiate research ethics consent from consent for data processing activities: when can an adolescent independently consent to research, or when is parental consent needed? Additional problems emerge when parents do not consent to research activities, but their adolescent children do. A serious ethical challenge can arise when the parents do not communicate with researchers, but the adolescent wishes to participate, especially when weak parental oversight is coupled with research on sensitive topics. We offer some guidance on what to keep in mind when conducting online research with adolescents and highlight possible ways that these issues can be dealt with in pan-European projects.