This study discusses the comparative application of child custody laws between developed and developing countries with a focus on the principle of "best interests of the child." Although this principle is recognized globally, its implementation varies widely across countries, influenced by social, cultural, religious, and economic factors. Developed countries such as the United States and Sweden tend to have more structured legal systems that support equality in custody, often adopting a joint custody model. In contrast, developing countries such as Indonesia and India face significant challenges in implementing this principle due to the influence of customary law, religion, and resource constraints. This study uses thematic and comparative analysis methods, and makes generalizations about the factors that influence the implementation of child custody in countries with different socio-economic backgrounds. The results of the study indicate that legal and social policy reforms, as well as increasing access to social services and legal education, are essential to improving the protection of children's rights in developing countries. Thus, recommendations focus on the need for international collaboration and increasing the capacity of courts to adopt best practices in child protection.