Parental knowledge about adolescents' activities is an identified protective factor in terms of adolescent adjustment. While research on parental knowledge has focused on adolescents' offline behavior, there is little empirical understanding of parental knowledge about adolescents' online behavior. This study investigates parental knowledge about adolescents' online activities and experiences with online risks, as well as the correlates of such knowledge. Building on former research, open communication and knowledge-generating monitoring practices are investigated as potential correlates of parental knowledge. Use is made of triadic data, relying on reports from children aged 13 to 18, mothers and fathers within the same family (N = 357 families; 54.9 % female adolescents). The results showed that parents have little knowledge about the occurrence of online risks and their children's online activities. While mothers did not have more accurate knowledge compared to fathers, they did perceive themselves to be more knowledgeable than fathers. Associations between parental knowledge and hypothesized correlates were tested by means of one-way ANOVA tests and stepwise logistic regression models. Limited evidence was found for associations with parents' accurate knowledge about the occurrence of online risks. Engagement in knowledge-generating monitoring practices was linked to mothers and fathers' self-perceived knowledge about their children's online activities. For mothers, open communication with the child was linked to self-perceived knowledge. The findings suggest that parents need to be more aware of the possibility that online risks might occur and that more research needs to be done in order to understand what parents can do to improve their accurate knowledge.