Y Combinator (YC) is an American technology startup accelerator launched in March 2005. It has been used to launch more than 4,000 companies, including Airbnb, Coinbase, Cruise, DoorDash, Dropbox, Instacart, Quora, PagerDuty, Reddit, Stripe and Twitch. The combined valuation of the top YC companies was more than $600 billion by January 2023. The company's accelerator program started in Boston and Mountain View, expanded to San Francisco in 2019, and was entirely online during the COVID-19 pandemic. YC is back in person as of the summer 2023 batch. Forbes characterized the company in 2012 as one of the most successful startup accelerators in Silicon Valley. History Y Combinator was founded in 2005 by Paul Graham, Jessica Livingston, Robert Tappan Morris, and Trevor Blackwell. From 2005 to 2008, one program was in Cambridge, Massachusetts, and one was in Mountain View, California. As Y Combinator grew to 40 investments per year, running two programs became too much. In January 2009, Y Combinator announced that the Cambridge program would be closed and all future programs would be in Silicon Valley. In 2009, Sequoia Capital led the $2 million investment round into an entity of Y Combinator to enable it to invest in approximately 60 companies a year. In 2010, Sequoia led an $8.25 million funding round for Y Combinator to further increase the number of startups the company could fund. In 2011, Yuri Milner and SV Angel offered every Y Combinator company a $150,000 convertible note investment. The amount put into each company was changed to $80,000 when Start Fund was renewed. In September 2013, Y Combinator began funding nonprofit organizations that were accepted into the program after testing the concept with Watsi. In 2014, Sam Altman replaced Graham as president of Y Combinator. In 2014, Y Combinator began a new deal for startups, offering $150,000 for 7 percent equity. In 2014, Altman announced a partnership with Transcriptic to provide increased support for Y Combinator's growing community of biotech companies. In 2015, he announced a partnership with Bolt and increased support for hardware companies. The YC Fellowship Program was announced in July 2015, with the goal of funding companies at the idea or prototype stage. The first batch of YC Fellowship included 32 companies that received an equity-free grant instead of an investment. In January 2016, Y Combinator changed the fellowship program, with participating companies receiving $20k investment for a 1.5 percent equity stake.