DC2NET Links

Do I need to ask the Webmaster of a site if I can put some of their content, i.e. articles, on my site if i credit them by putting a link to their site at the top of the article on my site? I know what the polite thing do do would be, and that would obviously be to ask. I’m wondering what the legal ramifications would be if I did not ask, but yet credited and linked to them.