According to Art. 6(1) GDPR, the processing of personal data is only lawful if at least one of the conditions listed therein is met. The most important legal bases include the data subject's consent (lit. a), the necessity for the performance of a contract (lit. b), and the pursuit of legitimate interests (lit. f). The choice of the appropriate legal basis and its careful documentation are decisive for every processing activity. If legitimate interest is to be used, a careful balancing test is required and must be documented.