The Rub' al Khali (الربع الخالي), or Empty Quarter, is the largest sand desert in the world, encompassing the southern third of the Arabian Peninsula, including southern Saudi Arabia, and areas of Oman, the United Arab Emirates and Yemen.

Still largely unexplored, and virtually uninhabited, the desert is a thousand kilometers (600 miles) long, and 500 km (300 mi) wide. Even the Bedouins only skirt the edges of the desert. Nonetheless, tour companies do exist which offer GPS-equipped excursions into the desert. The first documented Westerner to cross the Empty Quarter was Bertram Thomas in 1931.

With summer temperatures ranging from below freezing at night to over 60 degrees Celsius (140 F) at noon, and dunes taller than the Eiffel Tower - over 330 meters (1000 ft) - the desert may be the most forbidding environment on Earth. However, as nearly everwhere else, life flourishes here. Arachnids, rodents and plant life can all be found throughout the Empty Quarter. As an ecoregion it falls within the Arabian Desert and East Sahero-Arabian xeric shrublands.

Desertification has increased through the millennia. Before desertification made the caravan trails leading across the Rub' al Khali so difficult, the caravans of the frankincense trade crossed now virtually impassable stretches of wasteland, until about 300 CE. See for example the lost city of Ubar, which depended on such trade.