Sophie Tavakoli develops a novel doctrinal framework for the legal conceptualization of fiduciary relationships. Whereas German law has traditionally defined fiduciary relationships narrowly as property-related constructs grounded in proprietary rights, she demonstrates the methodological, doctrinal, and constitutional limits of this approach. She replaces the restrictive traditional definition with a broad modern understanding. Under this conception, fiduciary relationships constitute the fundamental legal structure for organizing personal relationships. All fiduciary relationships share the hallmarks of "protection and pursuit of another's interests by an independent third party." This overarching impression is further defined by four core characteristics: the protection of another's interests, strict orientation toward the beneficiary's interests, the fiduciary's legally mediated independence, and a qualified position of trust. For the first time, this approach allows for the incoroporation of personal, non-property interests into the concept of fiduciary relationships and thereby defines such personal relationships in legal terms. With regard to the physician-patient relationship, the fiduciary classification provides the doctrinal foundation for duties of disclosure, loyalty, and care, while linking medical-ethical notions of autonomy with civil-law doctrine. In the field of data trusts, the fiduciary framework enables a precise legal assessment of regulatory concepts situated between data protection and digitalization.