This article is devoted to the theory of interest as a criterion for distinguishing the abuse of civil law from unlawful behavior and from behavior that is legitimate but irrational. Good faith is an evaluative category that does not meet legal definitions, whether it is lawful conduct or misconduct. In this regard, the author suggests dividing lawful behavior into ordinary lawful (conscientious) behavior and socially encouraged behavior, which fully corresponds to the generally accepted concept of conscientious behavior. Since it is civil law that is characterized not only by the free will of subjects, the dispositivity and inadmissibility of arbitrary interference in private affairs, the principle of unhindered exercise of civil rights and the existence of such specific ways of regulating relations as legal custom, the previous behavior of subjects of law, it is precisely in civil law that factors that do not individually have legal significance began to acquire it in their entirety. The author notes that the development of the theory of interest is associated with the need to establish the intention to cause harm, the intention to achieve adverse consequences, or to balance conflicting interests with each other for their legitimacy. The “disadvantages” of using the criterion of “adverse consequences” are also noted - they may include any deterioration in the situation of third parties, which leads to a blurring of the category of “abuse of law”.