Alphabet's biggest and most profitable business unit, its search and ad business, is getting a new chief as the company faces growing legal and competitive challenges.

Nick Fox, a 21-year veteran and current head of product and design for Google Search and Assistant, will take the helm from Prabhakar Raghavan to lead its Knowledge & Information team, which spans its Search, Ads, Geo, and Commerce products. He previously reported to Raghavan.