The lawful bases for processing are set out in Article 6 of the GDPR. At least one of these must apply whenever you process personal data:
(a) Consent: the individual has given clear consent for you to process their personal data for a specific purpose.
(b) Contract: the processing is necessary for a contract you have with the individual, or because they have asked you to take specific steps before entering into a contract.
(c) Legal obligation: the processing is necessary for you to comply with the law (not including contractual obligations).
(d) Vital interests: the processing is necessary to protect someone’s life.
(e) Public task: the processing is necessary for you to perform a task in the public interest or for your official functions, and the task or function has a clear basis in law.
(f) Legitimate interests: the processing is necessary for your legitimate interests or the legitimate interests of a third party unless there is a good reason to protect the individual’s personal data which overrides those legitimate interests. (This cannot apply if you are a public authority processing data to perform your official tasks.)