Military service is mandatory for all South Korean men. In the ROK Navy, all members are volunteers, with enlisted seamen serving for 20-month periods; commissioned officers, warrant officers, and petty officers serving longer terms than those of enlisted seamen, or as career. Women can volunteer as officers, warrant officers, or petty officers. In 2001, six female ensigns, who had become commissioned officers through the Officer Candidate School, were assigned to serve on surface ships of the ROK Navy, for the first time.