信义义务最早出现在信托领域，属于衡平法上的制度。衡平法高度灵活，并且是以救济为导向的，衡平法上的格言是适用衡平法的总原则。信义义务的理解与适用离不开这些大的理论框架。信义关系与信义义务的逻辑关系并不清晰，常被用来相互定义。信义关系的类型繁多，既有公法上的也有私法上的。信义关系也处于不断的扩张中。英美法系的法官运用类推的方法，将争议案件类比已知的信义关系，如果具有相似性，即将其认定为新的信义关系，进而对其中的受托人课以信义义务。信义义务的理论基础可以从不同的角度来寻找。合同理论可以分为两种：描述性的合同理论与规范性的合同理论。主张合同理论的多为法经济学者。财产理论认为，信义义务的正当性在于规制受托人滥用其地位，违反受托目的处分财产。侵权理论很难说是一种论证信义义务正当性的理论，它只不过是把违反信义义务的法律后果归为一种特殊的侵权。公平理论、权力理论、脆弱性理论、信任理论都是从抽象的角度论证信义义务的合理性。针对违反信义义务的法律后果，普通法上的损害赔偿以及衡平法上的特有的救济方式都可以适用，法官拥有巨大的裁量权。衡平法上的特有救济方式主要有实际履行、禁制令、拟制信托、衡平法上的归入等等。在确定信义义务违反的程序中，一般实行举证责任倒置。信义义务与诚实信用有着密切联系，从根本上来讲，都可以看作私法中的信任机制，目的都是用法律制度维护社会信任。在适用信义义务时，诚实信用的内涵、适用方法可以为其提供借鉴。在英美法系，信义义务体现为一种行为标准，而诚实信用在大陆法系的法典中呈现为一般条款。信义义务为受托人设定的道德标准高于诚实信用，带有利他主义色彩。中国法上的信义义务体现为一种法定义务。具体而言，忠实义务体现为禁止性规范，或者说消极义务，内涵相对明确。勤勉义务体现为一般条款和不确定概念，需要法官进行价值填充与个案判断。归入在中国法上应当视作一种特殊的责任方式，具有一定的威慑与惩罚性功能。信义义务在中国法上存在推广适用的空间，譬如监护、医疗等领域。
Fiduciary duties, a concept in Equity, is pervasive in common law countries. This notion originates from trusts which are on the basis of feudal land system and use, and thus it possibly has property elements. Equity focuses on real intentions, instead of forms, and it is a remedy-driven system. It provides some reliefs different from those in common law, rather flexible and adjustable. Since the maxims are general principles of Equity, fiduciary duties should be consistent with them and other equitable doctrines.Fiduciary duties apply in fiduciary relationships, and generally each of them is used to define the other. There are various specific fiduciary relationships, in both public law and private law. Fiduciary relationships in private law consists of not only property ones, but also personal ones. Likewise, fiduciary relationship is an open-ended concept and expanded into fresh areas continuously. In common law, judges employ analogy to create the new types of fiduciary relationship, and then apply fiduciary duties to the fiduciaries in the new defined ones. The justification for fiduciary duties has no unanimity. Contractual theory can be summarized into two types: normative one and descriptive one. And law-economy analyzers prefer this theory. Property theory indicates the aim of fiduciary duty is to regulate the fiduciaries’ control of special property. Besides, critical resource theory is a peculiar branch of property theory. Technically, tort theory is not a separate justification, as it merely deems the outcome of the breach of fiduciary duties as tort. In the abstract, there are some other justifications, including equity theory, trust theory, power theory and vulnerability theory. In fact, some fiduciary relationships are formed on the control of property, and others purely arise out of private trust between parties without the management of property.For breaches of fiduciary duties, both common law and Equity provide various reliefs. In common law, damage is a main remedy. In Equity, remedies are far richer, including specific performance, injunction, constructive trust, accounting for profits and so on. All the remedies can be selected by judges at their discretion in specific cases. Generally, in the process of hearing, the burden of proof is reversed. Although fiduciary relationship is based on private trust, sometimes the liability form third party may be put forward.Fiduciary duties have some ties with bona fides, and obviously they have the same linguistic root. They can be seen as the trust mechanism in private law, designed to protect and enhance social trust. When applying fiduciary duties, judges can draw lessons from bona fides, especially in terms of the way for practice and interpretation. But these two concepts have some obvious differences. Fiduciary duty is some kind of standard of behavior in common law, while bona fide appears as a general clause in civil codes of Roman law countries. The moral foundation of the former is altruism, but the one of the latter is individualism of reasonable man in the market. In Chinese law, fiduciary duties are mostly written in commercial arenas. In positive laws, fiduciary duty is a statutory duty. Specifically speaking, the duty of loyalty appears as prohibitive norms or negative obligations and its content is comparatively clear. But the duty of care or diligence is a general clause and uncertain notion, and its content remains unclear and needs to be interpreted by judges in specific case. Accounting for profits is a special liability of torts, and it distinguishes from damages as it is a gain-based liability with function of deterrence and punishment. The operation of fiduciary duties in Chinese law can be shown via some typical cases. Fiduciary duties can be applied to wider domain in Chinese law, such as guardianship and healthcare.
