This study investigates social contact patterns in eight European countries to better understand how infectious diseases spread through close contact and respiratory transmission. A population-based survey using paper diaries collected data on 97,904 contacts recorded by 7,290 participants over one day. The data revealed that contact patterns are highly assortative by age, with individuals tending to mix with others of similar age. Physical contact was more common in home, school, and leisure settings than in workplace or travel settings. Contacts lasting at least an hour or occurring daily were more likely to involve physical contact, while shorter and less frequent contacts were nonphysical. The study found that children and young adults had the highest contact rates and that the initial phase of an epidemic would likely result in the highest incidence among 5- to 19-year-olds. The results provide a large-scale quantitative approach to contact patterns relevant for respiratory and close-contact infections, which can improve the parameterization of mathematical models used for disease control strategies. The study highlights the importance of age-specific mixing patterns and the role of physical contact in disease transmission. It also notes that contact diaries are a feasible method for collecting data on social contacts, although further validation with other methods is needed. The findings suggest that contact tracing efforts should focus on home, school, workplace, and leisure settings, as these are where the majority of contacts occur. The study underscores the need for further research to determine the epidemiological relevance of different types of contacts and to improve the accuracy of disease transmission models.This study investigates social contact patterns in eight European countries to better understand how infectious diseases spread through close contact and respiratory transmission. A population-based survey using paper diaries collected data on 97,904 contacts recorded by 7,290 participants over one day. The data revealed that contact patterns are highly assortative by age, with individuals tending to mix with others of similar age. Physical contact was more common in home, school, and leisure settings than in workplace or travel settings. Contacts lasting at least an hour or occurring daily were more likely to involve physical contact, while shorter and less frequent contacts were nonphysical. The study found that children and young adults had the highest contact rates and that the initial phase of an epidemic would likely result in the highest incidence among 5- to 19-year-olds. The results provide a large-scale quantitative approach to contact patterns relevant for respiratory and close-contact infections, which can improve the parameterization of mathematical models used for disease control strategies. The study highlights the importance of age-specific mixing patterns and the role of physical contact in disease transmission. It also notes that contact diaries are a feasible method for collecting data on social contacts, although further validation with other methods is needed. The findings suggest that contact tracing efforts should focus on home, school, workplace, and leisure settings, as these are where the majority of contacts occur. The study underscores the need for further research to determine the epidemiological relevance of different types of contacts and to improve the accuracy of disease transmission models.