Children migration has become an important phenomenon in recent years. In the European Union, 110,000 children have sought asylum between January and July 2015, among which 12,000 were unaccompanied (UNICEF, 2015, p.1). While there is a lack of exact data about migrant children in detention, the Global Detention Project reported in 2015 that “child migrants are routinely detained in many countries” (2015, p.18). The practice of detaining migrant children, in addition to having negative consequences on children’s health and development, also violates several children’s rights and state obligations under the Convention on the Rights of the Child (CRC). Greece has ratified the Convention, and has committed itself to give protection to children, with respect for the principle of the best interest of the child. However, Greece does not comply with the standards on the protection of migrant children. This article will focus on the situation of unaccompanied migrant children in Greece, and in particular on the issues of detention and guardianship.