Tim Cook Is Running Apple, But Not Imitating Steve Jobs

That relentlessness could be indispensable in the months ahead, because Mr. Cook may be tested as never before. He has been charged with running Apple's day-to-day operations while his boss, Steven P. Jobs, the company's visionary chief executive, is on medical leave.

Mr. Cook has done that twice before, briefly and successfully. Yet if Mr. Jobs's health does not improve, Mr. Cook could be on the job for a long time. And while Apple's succession plans are closely guarded,

Mr. Cook is widely believed to be the most likely candidate to permanently replace Mr. Jobs.