Thousands of South Sudanese are holding a huge street party to mark the first anniversary of their independence.
People are dancing and waving flags in the capital, Juba.
Nearby, there is an official ceremony where UN Secretary General Ban Ki-moon was due to be among the invited guests.
The BBC's Nyambura Wambugu, in Juba, says that many people feel it has been a good year, despite their country's problems and the lack of progress in their lives.
But she says it has also been a turbulent 12 months, with ethnic conflict in Jonglei State killing hundreds, conflict on the border with Sudan and a huge corruption scandal.
As well as Mr Ban, the leaders of neighbouring Ethiopia, Kenya and Uganda are set to attend the official celebrations at the mausoleum of South Sudan former rebel leader John Garang.
No senior Sudanese leaders were expected to turn up, with the two countries still disputing some border areas and how to share their oil wealth.
The celebrations featured a military parade, while a helicopter flew South Sudan's flag.
Sudan: A country divided
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. The residents of war-affected Darfur and South Sudan are still greatly dependent on food aid. Far more than in northern states, which tend to be wealthier, more urbanised and less reliant on agriculture.
