Abstract
This article presents the new regulation introduced to the Civil Code as Art. 4921, i.e. the declaration of non-performance, analyzed to date in literature as an anticipatory breach of contract. It specifies the subjective and objective scope of the regulation, its relation to debtor’s default, and analyzes legal consequences of the declaration of misperformance or partial performance. It determines also the legal nature of such declaration, its form, addressee, possibility to be revoked. It also examines similar categories of declarations. Finally, it evaluates the efficacy of introducing this institution to the Polish civil law.