Television networks will draw hordes of advertisers to New York City this week for their annual bonanza of presentations and parties, a decades-old tradition known as the upfronts that is meant to dazzle marketers and loosen their purse strings.

New shows and top talent will be pitched from the stages of Carnegie Hall and Lincoln Center, followed by lavish evening affairs where marketers can eat lobster rolls and snag selfies with network stars. The fanfare will kick off weeks of negotiations, with networks aiming to get advertisers to commit to billions of dollars in spending for the year ahead.

But beneath the sparkle and the canapés, the networks are also navigating a serious advertising upheaval. Ratings are on the decline, especially among young people, some of whom don’t even own televisions. It’s hard to keep up with the many devices and apps people now use to watch shows. And there is a host of material from Silicon Valley that is competing for viewers’ attention, including Google’s YouTube, Facebook and Netflix. It all adds up to a precarious situation for broadcast TV.