The concept of the best interests of the child, as one of the fundamental principles of international law and especially in the Convention on the Rights of the Child, expresses attention to the welfare, security and mental health of the child in all judicial and administrative decisions. This article, by analyzing custody, attempts to examine the concept of "the best interests of the child" from three legal, jurisprudential and psychological perspectives and with a descriptive-analytical method. Iranian law is no exception to this and finally, in accordance with the aforementioned convention, the best interests have been included as a principle preceding all matters in the Family Protection Law. Extensive research has been conducted in the international arena on the best interests of the child, which has led to the compilation of many books with the efforts of the Committee on the Rights of the Child and UNICEF in interpreting the provisions of the convention to identify the elements of the best interests. Consequently, the court should make decisions on custody based on legal and pathological assessments, in particular an examination of the child’s emotional and psychological state, the background and capabilities of the biological parents, and the quality of the new family environment. Only if it is proven that returning to the biological family would be in the child’s best interests and would not pose psychological or social risks to the child, can custody be restored. Otherwise, maintaining the status quo or adopting alternative arrangements should be preferred in order to protect the child’s rights and mental health.