Lihou Reef and Cays is the largest atoll by total size in the Coral Sea. It is a part of the Coral Sea Islands Territory. The atoll is U-Shaped, with an opening in the Southeast. The size of the lagoon is about 100 by 30 km², with an area of 2500 km², which makes it the atoll one of the largest in the world. Nevertheless, the total land area of the islets and cays is only 0.91 km². The lagoon is up to 60 m deep.

The atoll and the waters around were declared, on August 16, 1982, the Lihou Reef National Nature Reserve, with a total area of 8440 km², to protect wildlife.

There is a total of 18 islets and cays, 9 each on the Northern Rim and on the Southern Rim. The range in size from 0.7 ha to 17 ha.

Northern Rim

The islets on the Northern Rim are, from Southwest to Northeast (clockwise):

1. Juliette Cay
2. Kathy Cay
3. Lorna Cay
4. Little Margaret Cay
5. Margaret Cay
6. Turtle Islet
7. Middle Cay
8. Observatory Cay
9. Licklick Cay

Only Turtle Islet and Lorna Cay are vegetated. Turtle Islet reaches a height of 6 m, and Observatory Cay 2 m. There is an automatic weather station on Turtle Islet.

Southern Rim

The islets on the Southern Rim are, from Northeast to Southwest (clockwise):

1. Anne Cay ("No 1 Cay")
2. Betty Cay ("No 2 Cay")
3. Carol Cay ("No 3 Cay")
4. Dianna Cay ("No 4 Cay")
5. Fanny Cay ("No 5 Cay")
6. Edna Cay ("No 6 Cay")
7. Helen Cay ("No 7 Cay")
8. Georgina Cay ("No 8 Cay")
9. Nellie Cay ("No 9 Cay")

Only Anne Cay, Georgina Cay and Nellie Cay are vegetated. Just Northeast of Anne Cay is the Herald Passage into the lagoon.