The Biggest AI and SEO News in May 2026: Google I/O, the May Core Update, and a Billion-User Milestone

May 2026 may have been the most consequential month in search since the AI era began. Google I/O delivered the biggest Search overhaul in 25 years, AI Mode crossed a billion monthly users while AI Overviews hit 2.5 billion, a broad core update landed within 48 hours of the keynote, and Google published its first official guide to optimizing for AI features — all while new data showed AI Overviews cutting click-through rates by more than half. Here's everything that happened, sourced to the original reporting, plus the one thing that ties it all together: search is now an AI-answer system at scale, and the rules for being surfaced inside it are being written in real time.