One of the most common misconceptions startups have about applying to Y Combinator is thinking that they are too early or too late.
We have funded companies with only an idea; we’ve funded companies with millions of users, millions of dollars of revenue, and millions of dollars raised. In nearly every case, the founders tell us they got a lot of value out of Y Combinator, and that the equity we take more than pays for itself. We continue to add more features to YC to help later-stage companies--i.e., our alumni. And of course, the YC alumni network is helpful at any stage.
We think we can help companies at any stage up to Series B, probably, and perhaps even later (but it gets difficult to convince later stage investors at that point, even if the founders themselves want to do YC). We encourage prospective founders to reach out to alumni to get their opinion on if YC would be a good fit.
Even if you haven't gotten in previously, you should apply again. Many of our successful founders applied multiple times before we funded them (including Drew Houston of Dropbox).
You also don’t need to have a pre-existing relationship with us. A lot of founders seem to think they need to figure out a way to meet with us or talk to us outside of the application process because VCs traditionally don’t fund companies they meet without an introduction. This is part of our model; we’re willing to do the work to look at thousands of companies that come to us without an introduction. In fact, we love doing so—many (perhaps most) of the best companies get started by unknowns.