Allen was among the world's wealthiest individuals and ranked 44th on Forbes' 2018 list of billionaires with an estimated net worth of more than $20 billion.

Through Vulcan, Allen's network of philanthropic efforts and organisations, the Microsoft co-founder supported research in artificial intelligence and new frontier technologies. The group also invested in Seattle's cultural institutions and the revitalization of parts of the city.

Allen owned two professional sports teams, the NFL Seattle Seahawks and NBA Portland Trailblazers

Vulcan CEO Bill Hilf said, "All of us who had the honor of working with Paul feel inexpressible loss today."

"He possessed a remarkable intellect and a passion to solve some of the world's most difficult problems, with the conviction that creative thinking and new approaches could make profound and lasting impact," Hilf said in a statement.

Allen revealed that he had started treatment for non-Hodgkin's lymphoma, the same type of cancer he was treated for in 2009. The illness was one of the reasons Allen left Microsoft in 1983 however he defeated it.

Bill Gates, who co-founded Microsoft with Allen, said that "personal computing would not have existed without him":

"I am heartbroken by the passing of one of my oldest and dearest friends, Paul Allen. From our early days together at Lakeside School, through our partnership in the creation of Microsoft, to some of our joint philanthropic projects over the years, Paul was a true partner and dear friend. Personal computing would not have existed without him.

Allen wasn't content with starting one company. He channelled his intellect and compassion into a second act focused on improving people's lives and strengthening communities in Seattle and around the world. He was fond of saying, "If it has the potential to do good, then we should do it." That's the kind of person he was, Gates said.

“Paul loved life and those around him, and we all cherished him in return. He deserved much more time, but his contributions to the world of technology and philanthropy will live on for generations to come. I will miss him tremendously."

Current Microsoft CEO Satya Nadella said Allen made "indispensible" contributions to Microsoft and the technology industry. Nadella also said he learned a lot from Allen and will continue to be inspired by him.

"As co-founder of Microsoft, in his own quiet and persistent way, he created magical products, experiences and institutions, and in doing so, he changed the world," Nadella said in a statement.

The shy and retiring former Microsoft CEO Steve Ballmer remembered Allen as "truly wonderful, bright and inspiring person."

Apple CEO Tim Cook called him a "pioneer" and a "force for good."

Salesforce CEO Marc Benioff said he was saddened by Allen's passing who he called great leader in tech.

“He was a man of all seasons who fully enjoyed his life and wealth yet also gave back to the world at scale. I was especially impressed with how he took care of the Oceans. May the one who brings peace bring peace to all.”
Amazon CEO Jeff Bezos praised his "relentless" push forward in technology:

“His passion for invention and pushing forward inspired so many. He was relentless to the end.”