Mahathir Mohamad has been sworn in as prime minister of Malaysia after his shock election victory, 15 years after he stood down.
After taking the oath at the Istana Negara palace in Kuala Lumpur, he told reporters his focus would be on the country's finances.
The former strongman has become, at 92, the world's oldest elected leader.
He came out of retirement and defected to the opposition to take on and beat former protege Najib Razak.
His historic win ousted the Barisan Nasional (BN) coalition, which has been in power since independence in 1957.
People cluster round to watch Mahathir Mohamad sworn in as Malaysia’s 7th PM (he was its 4th too). Cheers when he’s finished - they really are delighted to have him back. But what does his disparate coalition government do next? Is he the man the cleanse the Augean stables? pic.twitter.com/8vRhujep8J— Jonathan Head (@pakhead) May 10, 2018
What other policies did he outline?
Dr Mahathir took his oath of office before the king, Sultan Muhammad V. The new prime minister was accompanied by his wife, Siti Hasmah Mohammed Ali.
Addressing the media, he pledged that Malaysia would remain a "friendly trading nation" and he would work to keep the currency, the ringgit, as "steady as possible".
He said he would seek the return of millions of dollars lost in a corruption scandal at 1Malaysian Development Berhad (1MDB), a state investment fund set up by Mr Najib.
"We believe that we can get most of the 1MDB money back," he was quoted as saying by Reuters news agency, adding: "We have to increase the confidence of investors in the administration."
He renewed his promise to seek to have his former deputy prime minister, Anwar Ibrahim, released and pardoned.
When Dr Mahathir was previously in power, Anwar was jailed for corruption and sodomy after calling for economic and political reforms.
He was released in 2004 but jailed again under Mr Najib in 2015. He is currently due for release next month.
What was the scale of the victory?
Investment analyst Aninda Mitra told Reuters news agency the shock of the election outcome had been as great as "Brexit and the Trump election".
Official results show Dr Mahathir's Pakatan Harapan (Alliance of Hope) coalition secured 113 of the 222 seats being contested, including some which have only ever been held by the government. BN took 79 seats.
Rising living costs and long-running allegations of corruption had weighed heavily on many voters and saw them peel away from Mr Najib and his once unshakeable coalition.
How are people taking Mahathir's return?
"We feel so united tonight," student Abdul Aziz Hamzah, 24, told AFP news agency in the crowd of jubilant supporters outside the palace. "Mahathir is so insightful and experienced because he's been here before."
Fahmi Fadzil, an MP from the winning coalition, told the BBC Dr Mahathir inspired excitement about Malaysia's future.
"Today he has created, along with other Pakatan Harapan leaders, a resurgence in faith in Malaysia," he said.
"The polling centres that I visited, there was a sense of euphoria among people who cast their vote. Members of the press that I had met, some of them said that they cried when they heard that we had won.
"There is very real change happening in Malaysia. Finally we can believe in the country again."
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How are Mahathir's earlier years in office remembered?
He was prime minister, at the head of the BN coalition, for 22 years, from 1981 until he stepped down in 2003.
Under his leadership, Malaysia became one of the Asian tigers - the group of countries which saw their economies expand rapidly in the 1990s.
However, he was an authoritarian figure who used controversial security laws to lock up his political opponents.
Where did Najib fall down?
Dr Mahathir was also a mentor to Mr Najib, who became prime minister in 2008.
But Mr Najib was accused of pocketing some $700m (£520m) from the 1MDB but vehemently denied the allegations and was cleared by Malaysian authorities.
The fund, meanwhile, is still being investigated by several countries.
Mr Najib was also accused of stifling Malaysian investigations by removing key officials from their posts.
Those allegations led to Dr Mahathir's surprise defection in 2016 from BN to join the Pakatan Harapan, saying he was "embarrassed" to be associated "with a party that is seen as supporting corruption".
Then in January, he said he would run for the leadership again.
But despite his historic win, uncertainty hangs over his tenure. Prior to his win, he intended to govern for two years before stepping down.
'Mountain of challenges'
By Jonathan Head, BBC South East Asia correspondent
This morning Malaysia has woken to an entirely new situation, the first transfer of power in its history, albeit to a very familiar leader. But there are huge unknowns. How willingly will BN, the coalition which has, in various forms, run the country since independence and embedded itself into all areas of governance, relinquish power?
How well will a disparate coalition, united largely by their desire to oust Najib Razak, work together in government? How smoothly will the plan to gain a pardon for imprisoned opposition leader Anwar Ibrahim, and then for Mr Mahathir to hand the premiership to him within two years, actually proceed? And how will they treat Mr Najib, and his high-spending wife, both accused of greed and corruption?
After all the jubilation over an impressive act of defiance by Malaysian voters, there is a mountain of challenges to face.