The answer to this question may depend on the country in which you are traveling. In some cases, both parents may have to approve international travel, while in others only one parent may need to give permission. It is always best to check with your specific airline or travel agent to find out what their requirements are.
In general, when minors are traveling internationally without both parents or a legal guardian, it is recommended that they carry a notarized letter of consent from the absent parent(s). This letter should state that the parent(s) gives permission for the child to travel and provide contact information in case of an emergency. The child should also carry copies of the parent’s passport and birth certificate.