Apple is expected to release the next version of its mobile operating software, the iOS6, as well as spruce up its MacBook lineup.
Long lines marked the beginning of the week-long annual forum, where Apple developers rub shoulders with employees, test the latest products and software, and connect with peers.
More than ever, Apple finds itself in a pitched battle with Google: in smartphones, cloud computing, and a never-ending competition to attract the best software developers.
That is crucial as Apple looks to draw users deeper into its applications ecosystem.
Cook told the audience that customers have downloaded more than 30 billion Apple apps so far, choosing from more than 650,000 apps -- the largest library in the industry.