Some world leaders serve a set term and others go on to rule for decades.
Here is a list of current world leaders who have held onto power the longest. The list excludes monarchs.
Equatorial Guinea
Teodoro Obiang Nguema Mbasogo, 77, has been president of the central African nation for some 40 years after toppling his uncle in a 1979 coup. He tops the list of the world's current longest-ruling leader.
His government has been besieged by allegations of corruption and mismanagement.
The President of Equatorial Guinea, Teodoro Obiang Nguema meets with Prime Minister of Ethiopia Hailemariam Desalegn (not seen) at Addis Ababa Bole International Airport in Addis Ababa, Ethiopia on February 21, 2018. (Photo by Minasse Wondimu Hailu/Anadolu Agency/Getty Images)
His son, Teodoro Nguema Obiang Mangue, is also the country's vice president. In 2016, his collection of 25 exotic cars were seized during a corruption investigation and sold at a charity in Switzerland for over $27 million.
Cameroon
Paul Biya, 86, took office in 1982 after a seven-year stint as prime minister. His presidency has faced threats from opposition parties and increased violence in the north and west portions of the country, where separatists have fought for a separate independent state.
President of Cameroon Paul Biya with Chinese President Xi Jinping (not pictured) attend a signing ceremony at The Great Hall Of The People on March 22, 2018 in Beijing, China. (Photo by Lintao Zhang/Getty Images)
Cameroon also faces threats from Boko Haram militants across the border in Nigeria.
Cambodia
Cambodia's Prime Minister Hun Sen began his fifth term in 2018, cementing his status among the world's longest rulers. The 67-year-old has been in power for 34 years, strong-arming his way through his country's political system.
He fled to Vietnam during the regime of Pol Pot, when an estimated two million Cambodians were killed. He returned in 1979, eventually becoming prime minister in 1985 in a Vietnamese-backed single-party communist government and led Cambodia through a civil war against the radical communist Khmer Rouge.
Cambodia's Prime Minister Hun Sen adjusts his glasses during the 25th International Conference on The Future of Asia on May 30, 2019 in Tokyo, Japan. (Photo by Tomohiro Ohsumi/Getty Images)
He cracked down on critics and opponents after the 2013 general election showed declining support for his ruling party.
Uganda
Longtime President Yoweri Museveni is expected to return as leader for his sixth term after a court endorsed a law removing an age limit for the country's president.
Prime Minister Theresa May meets President Yoweri Museveni of Uganda during the London Conference on Somalia at Lancaster House on May 11, 2017 in London. (Photo by Hannah McKay - WPA Pool/Getty Images)
Museveni, 75, took power by force in 1986 and declared himself president. He's been re-elected several times since, though international observers have noted issues with the polling process in several elections.
Iran
Iran's second Supreme Leader Ayatollah Ali Khamenei wields exceptional power over the country. Before taking the position in 1989, he served as the president, a largely ceremonial role.
In this photo released by an official website of the office of the Iranian supreme leader, Supreme Leader Ayatollah Ali Khamenei attends a meeting with Iranian officials, participants of the 31st International Islamic Unity Conference and ambassadors from Islamic countries, in Tehran, Iran, Wednesday, Dec. 6, 2017. Khamenei condemned President Donald Trump's imminent recognition of Jerusalem as the capital of Israel. (Office of the Iranian Supreme Leader via AP)
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Khamenei, 80, is considered Iran's most powerful figure and has control over its branches of government, military and the media. He is the second-longest serving autocrat in the Middle East and the second-longest serving leader in Iran in the last century, after the Shah Mohammed Reza Pahlavi.
Other mentions:
Deceased North Korean founder Kim Il-sung ruled his rouge state for 46 years before dying in 1994 while in office.
Albania's Enver Hoxha remained in power for four decades before his 1985 death, and Fidel Castro came to power in the 1959 Cuban revolution. The deceased leader handed the presidency to his brother Raul in 2008.
The Associated Press contributed to this report.