Y Combinator, founded by Paul Graham in 2005, popularized a Silicon Valley accelerator model built around seed funding and startup equity.
The first Y Combinator program ran in Cambridge, Massachusetts, before the organization became closely associated with Silicon Valley and moved its main operations to Mountain View, California. Its early batches gave founders a small amount of funding, intensive advice, and access to a network of investors. The program culminated in a public Demo Day where startups presented to potential backers.
Y Combinator became influential because it standardized a repeatable path for very young companies: apply, join a short cohort, build quickly, talk to users, and raise additional capital. Paul Graham co-founded it with Jessica Livingston, Robert Morris, and Trevor Blackwell. The accelerator has backed companies including Airbnb, Dropbox, Stripe, and Reddit, but it is not the same organization as Techstars or 500 Global.