Behavioural targeting uses a user's web browsing history to target relevant ads. It works by tracking data from internet service providers, websites visited, user profiles, and other networks to categorize users and their potential interests. While this can improve ad relevance and returns for advertisers, it also raises privacy concerns. Several companies have implemented behavioural targeting in different ways online and on mobile devices, with some attempting to address privacy issues through anonymity and user preferences. However, regulators and privacy advocates continue to scrutinize industry practices.