An apprentice is any cat in a clan that is six moons or older, and is currently being mentored by an older clan cat. Kits become apprenticed to a mentor that is suited to help them learn the ways of the clan after they reach their sixth moon. Apprentices learn how to hunt, patrol, and fight for their clan. Occasionally a mentor will assign their apprentice a certain hunting or battling assessment, and they are watched carefully to make sure they are learning. After a certain amount of time, usually 2-3 moons or when the mentor believes they are ready, an apprentice becomes a warrior. As their first night as a warrior, his or she must watch over the clan in his/her vigil.
There is also medicine cat apprentices. These are cats-- whom can be at any age-- who are chosen by StarClan to learn the ways of a medicine cat. They do not sleep in the apprentice den; they sleep in the medicine cat den with their mentor. Instead of staying up for their vigil, a medicine cat apprentice gets his or her full medicine cat name at the StarClan meeting place.