Getting discovered as a pop star used to be a complicated process.
You would build up a local fan base through concerts at pubs and clubs, record demo tapes and mail them to record labels, who would either reject or accept them.
You would then record a song and hope it would get played on the radio, and then hope listeners would like it enough to buy the single.
It's not the case any more, thanks to video-sharing sites like YouTube.
While millions of aspiring performers upload their videos in the hope of getting discovered, a select few break through to make it to the big time.
Whether it's a conventional teen pop star like Justin Bieber or a more unlikely performer like Psy, we've compiled a list of the biggest artists to make it through YouTube.