Y Combinator was founded in 2005 by Paul Graham, Jessica Livingston, Robert Morris, and Trevor Blackwell. The firm is classified as a technology startup accelerator—meaning that it's investors focus on seed stage companies, investing relatively small amounts of capital while offering guidance and business counsel. Y Combinator is selective in choosing what companies to support, announcing two "batches" of companies each year that the firm's investors believe have a strong future as high growth businesses. When a company grows beyond the seed stage, Y Combinator may continue to offer financial and strategic support through a separate program and fund that the firm created for this purpose. The firm has earned a reputation as being one of the most successful startup accelerators in Silicon Valley.
Y Combinator reports that since 2005, it has invested in "over 3,000 companies that are worth over $400B combined." An impressive number of large companies received support from Y Combinator at some point in their development, including Airbnb, Stripe, Instacart, Dropbox, Coinbase, DoorDash, and Reddit. Other portfolio companies that went on to go public through an IPO include GitLab, PagerDuty, and Weave.