None of the US Pacific Fleet aircraft carriers were in Pearl Harbor when the attack came. This has been alleged by some to be evidence of advance knowledge of the attack; the carriers were supposedly away so as to save them (the most valuable ships) from attack.
In fact, the two carriers then operating with the Pacific Fleet,
Enterprise and
Lexington, were on missions to deliver fighters to Wake and Midway Islands. (The third,
Saratoga, was in routine
refit at Bremerton, in the
Puget Sound shipyard.) These assignments sent the carriers west, toward Japan and the
Kido Butai, lightly escorted. At the time of the attack,
Enterprise was about 200 miles (370km?) west of Pearl Harbor, heading back. In fact,
Enterprise was scheduled to be back on December 6th, but was delayed by weather. A rescheduling had her estimated time of arrival as 7:00, almost an hour before the attack, but she was also unable to make this schedule.
Furthermore, at the time, aircraft carriers were classified as fleet scouting elements, and hence relatively expendable; they were not
capital ships. The most important vessels in naval planning even as late as Pearl Harbor were battleships (per the
Mahanian doctrine followed by both the U.S. and Japanese navies at the time). As the only surface strike power remaining available in the Pacific fleet, carriers became the Navy's most important ships following the attack, and their work, added to the results of the Pearl Harbor Raid, and the sinking of the Prince of Wales and Repulse, effected a sea change in worldwide Naval thinking.
Thanks for the link. 
