Breach of fiduciary duty is a cause of action or claim in civil litigation (lawsuits) that provides the legal basis for a person or entity to recover its damages/losses when there is a special relationship based on trust and confidence (attorney and client or trustee and beneficiaries) and the party who owes the fiduciary duty breaches its duty of loyalty (conflict of interest) or duty of care (informed judgment in decision-making).

In Delaware, a breach of fiduciary duty occurs when an individual or entity, who is obligated to act in the best interest of another party due to a special trust relationship, fails to do so. This breach can involve a conflict of interest or a failure to make informed decisions, which are core components of the duty of loyalty and the duty of care, respectively. Fiduciary relationships are common in contexts such as corporate governance, where directors owe duties to the corporation and its shareholders, and in the management of trusts, where trustees owe duties to beneficiaries. When a breach of fiduciary duty is alleged, the affected party may file a civil lawsuit to recover damages. Delaware courts have developed a substantial body of case law on fiduciary duties, particularly in the corporate context, given the state's prominence in corporate law. Remedies for breach of fiduciary duty in Delaware can include monetary damages, equitable relief such as injunctions, and, in some cases, the imposition of constructive trusts or the unwinding of transactions.