Payment initiation services offer a technically innovative way to make payments online, based on online banking. Access to the account by third-party service providers raises issues of security, data protection, and competition. These aspects have led to regulation by the European legislator, who established a legal framework for payment initiation services for the first time under the Second Payment Services Directive (PSD2). Elisabeth Meister addresses the national implementation law at the interface between civil and supervisory law. Starting with a fundamental classification of the phenomenon of payment initiation services in payment transactions, she first undertakes an in-depth systematization of the facts and the various forms of payment initiation services, before turning to the private law relationships between the actors associated with these services.