Video advertising has started on the YouTube website, its owner - internet giant Google - has confirmed.
YouTube has more than 130 million unique monthly users
Google said it had designed the way the adverts work on the video-sharing website to be as unobtrusive and undisruptive as possible.
The adverts will begin 15 seconds after a user has started to watch a video, but only on 20% of the screen window.
Google said the advert would then disappear within 10 seconds if the user had not clicked to watch it.
While YouTube already carries advertising banners, the introduction of video adverts is Google's first big push to increase revenues on the video-sharing website, which it bought for $1.65bn (£831m) last November.
"The philosophy at YouTube is pretty much core to what we at Google do generally, which is that all the ads we serve need to provide value to the end user," said Eileen Naughton, Google's director of media platforms.
The first video advertisers keen to tap into YouTube's more-than-130 million unique monthly visitors include Warner Music, News Corporation, 20th Century Fox and New Line Cinema.
The adverts will be connected to music videos and other selecting postings.
According to market expectations, the value of video advertisements on websites will soar to $4.3bn a year in the US alone by 2011.