Once a person is online, personal data are collected, analyzed and processed. This thesis gives an introduction to tracking on the Internet. By using examples from known companies, it shows which data are collected from whom and for what reasons. The terms first-party tracking and third-party tracking are explained. Common tracking technologies are presented, such as HTTP cookies, etags, web bugs, history sniffing, fingerprinting or tracking via social media plug-ins or the Internet provider. Legal, self-regulating and technical measures for data protection and privacy are discussed. Furthermore, the tracking and profiling techniques of ten popular Austrian websites are analyzed and the results are presented. The thesis concludes with an example of best practices.