British Prime Minister Boris Johnson is among the world leaders congratulating Joe Biden on the news that he will be the next president of the US.
He added: "The US is our most important ally and I look forward to working closely together on our shared priorities from climate change to trade and security."
Former US presidents Barack Obama and Bill Clinton have congratulated Mr Biden and Ms Harris, as has Ireland's prime minister, which will mean much to the president-elect, given his Irish ancestry. Here are some of the messages from around the world...
The former president said he "could not be prouder", adding: "We're fortunate that Joe's got what it takes to be president and already carries himself that way.
"Because when he walks into the White House in January, he'll face a series of extraordinary challenges no incoming president ever has - a raging pandemic, an unequal economy and justice system, a democracy at risk, and a climate in peril.
"I know he'll do the job with the best interests of every American at heart, whether or not he had their vote. So I encourage every American to give him a chance and lend him your support."
He added: "Our democracy needs all of us more than ever. And Michelle and I look forward to supporting our next president and first lady however we can."
"America has spoken and democracy has won.
"Now we have a president-elect and vice president-elect who will serve all of us and bring us all together.
"Congratulations to Joe Biden and Kamala Harris on your momentous victory!"
Prime Minister Justin Trudeau said: "Canada and the United States enjoy an extraordinary relationship - one that is unique on the world stage.
"Our shared geography, common interests, deep personal connections, and strong economic ties make us close friends, partners, and allies.
"We will further build on this foundation as we continue to keep our people safe and healthy from the impacts of the global COVID-19 pandemic, and work to advance peace and inclusion, economic prosperity, and climate action around the world.
"I look forward to working with president-elect Biden, vice president-elect Harris, their administration, and the United States Congress as we tackle the world's greatest challenges together."
Chancellor Angela Merkel said she was looking forward to "future cooperation" between the two countries, adding: "Our transatlantic friendship is irreplaceable if we are to master the great challenges of our time."
The German leader had enjoyed a good working relationship with Barack Obama during his presidency but the relationship between her and Donald Trump was often strained.
Poland was one of the US's closest allies under Donald Trump but President Andrzej Duda said his country was determined to maintain a "high-level, high-quality... partnership" under Mr Biden.
Prime Minister Pedro Sanchez said: "Congratulations Joe Biden and Kamala Harris. We wish you good luck and all the best.
"We are looking forward to cooperating with you to tackle the challenges ahead of us."
President Emmanuel Macron said: "We have a lot to do to overcome today's challenges. Let's work together."
Taoiseach Micheal Martin said: "Ireland takes pride in Joe Biden's election, just as we are proud of all the generations of Irish women and Irish men and their ancestors whose toil and genius have enriched the diversity that powers America," he said
"I also wish to congratulate Kamala Harris on her election as vice president and recognise the tremendous significance of her election to the role."
The reaction in Iran was more focused on the departure of Mr Trump, rather than the arrival of Mr Biden in the White House.
Hesameddin Ashena, an adviser to Iran's president, said on Twitter that Iranians "stood their ground bravely until that coward left".
Mr Trump's administration had overseen a worsening of relations between the two countries, especially after the US pulled out of Tehran's nuclear deal in 2018 and re-imposed sanctions.
Iran's ambassador to the UK, Hamid Baeidinejad, tweeted: "Finally, the political life of a man who only spreads hatred is over."
Mr Biden has promised to re-join the nuclear deal if Iran complies with it again.
Prime Minister Scott Morrison said: "Australia wishes you every success in office. The Australia-US alliance is deep and enduring, and built on shared values," he said.
"I look forward to working with you closely as we face the world's many challenges together."
Prime Minister Jacinda Ardern said: "New Zealand will continue to work side-by-side with the United States on the issues that matter to both of us, including the prosperity, security, and sustainability in the Indo-Pacific and Pacific Island regions.
"The campaign by the president-elect has also shown the shared interests we have in addressing global challenges like COVID-19 and climate change."
And in the UK, Sir Keir Starmer, Labour Party leader
He said Mr Biden had run "a campaign on the values that we in the United Kingdom share - decency, integrity, compassion and strength".