Steve Jobs, the co-founder and head of Apple Inc., the company behind the wildly popular iPhones, iPads, and iPods, has resigned. In a letter, Mr. Jobs told the Apple Board of Directors that he could no longer meet his duties and expectations as the company's Chief Executive Officer. Jobs had battled pancreatic cancer and received a liver transplant. He has been on medical leave since January. Apple said in a statement that Mr. Jobs had served Apple with extraordinary vision and leadership. The company said, Mr. Jobs has been elected as Chairman of the Board of Directors and will, in that role, continue to contribute unique insights, creativity and inspiration. The statement said, Tim Cook will take over as Apple's new top executive at the recommendation of Jobs. Cook had previously served as the company's chief operations officer. Experts say the technology giant, Apple, will now face big tests in staying ahead of competition and achieving success without Steve Jobs in charge. In a surprise announcement, Apple on Thursday said Jobs has resigned as company CEO and named Chief Operating Officer Tim Cook as his successor. Jobs will now be the chairman of Apple's board. Media reports said the executives that will now run Apple without Jobs will face big tests of whether they can still excel in highly competitive businesses that often have small profit margins.