The rank structure for U.S. Navy officers begins with ensign and ends with admiral. Beginning with the lowest, the ranks are ensign, lieutenant junior grade, lieutenant, lieutenant commander and commander. They continue increasing to captain, rear admiral lower half, rear admiral upper half, vice admiral and admiral.
An ensign is the equivalent of a second lieutenant in the U.S. Army. Ensigns are eligible for promotion after two years of service. A lieutenant junior grade is the equivalent of a first lieutenant in the U.S. Army. Lieutenants junior grade are available for promotion after four years of service. A lieutenant is the equivalent of a captain in the U.S. Army. Lieutenants are eligible for promotion after nine to 11 years of service. Officers at every rank compete with each other for promotion to the next rank.
Lieutenant commander, commander and captain are classified as control grades. The number that the Navy can have in each of these grades is set by law and cannot be exceeded. Officers may only be promoted to these grades when there is a vacancy, even if they have met the required time in service.
Admirals are the equivalent of U.S. Army generals. The rank of fleet admiral exists above the rank of admiral but is only used during wartime.