Actually the EU is non-canon. List posted a while back by LucasFilms, Ltd to answer this question once the EU literatue began appearing:
ONLY the films, film novelizations, and radio drama adaptations are canon. Artwork and other such related materials used to make the films, provided it does not contadict the final versions as seen in the films, may also be considered canon. The radio dramas, novels, and presumably concept material serve to fill in the gaps left by the movie (the Derra IV Massacre is mentioned in the ESB radio drama, making Commander Narra's existence and death canon, however his participation at Yavin is NOT canon because it's mentioned in none of those sources) and there's a strict hierarchy in the event of disagreements (which happens due to changes in script/etc during poduction): The radio dramas are overridden by the novels, and the films override EVERYTHING (therefore the Rebel Medical Frigate in RotJ is one of the small Nebulon-B Escort Frigates as shown on-screen, NOT one of the winged
Liberty-class Star Cruisers referred to in the radio drama and novelization).
The EU and various games, technical manuals and other resources are considered official, but NON-canon as long as they don't directly contradict the films. The canon sources override the EU/games/other lesser offical souces (the size of the
Executor being the most obvious).
Anything NOT made by LucasArts/LucasFilm or anything that directly contradicts the canon sources falls under the category of apocrypha. This includes fanfiction/films, as well as official literature or productions that contadicts or is overruled by the movies (which includes the novel Splinter of the Mind's Eye, written before Empire Strikes Back was conceived, and the Star Wars Holiday Special). It's never made clear whether elements of EU literature which contradicts the canon renders the entire work as apocrypha or not (IE: Zahn's trilogy sets the Clone Wars exactly 50 years prior to ANH, directly contradicting the setting as presented in the Prequel Trilogy. However does this make the entirety of the Thrawn Trilogy apocrypha?)
Lucas can therefore contradict the EU literature all he wants. It's not a part of the canon and he has no obligation to include it in the history of the Star Wars Universe and numerous examples exist of the prequels directly contradicting the EU (Threepio's age and origin, Owen Lars is NOT Obi-Wan's brother, the Death Star was NOT designed by Bevel Lemelisk in the Maw).
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[This message has been edited by The Saxman (edited May 27, 2003).]