Prediction markets have a vanity metric problem: trading volume. Volume looks like traction, but it can spike around major events and count the same capital repeatedly as traders rotate positions. Open interest tells a much cleaner story. As of May 1, tracked prediction markets held roughly $1.11B in OI. Kalshi: ~$630.7M Polymarket: ~$449.9M Together, they accounted for almost 98%. Everyone else was below $15M. That changes how I read the category. This isn't ten platforms competing on equal footing. It's two liquidity centers followed by a long tail of increasingly specialized products. @Kalshi is winning with regulated rails and brokerage-like UX. @Polymarket owns the crypto-native position. Smaller players are differentiating around specific environments: exchange-style crypto markets, macro contracts, or prediction markets embedded directly into existing trading venues. The broader lesson is simple: activity is not the same as depth. You can subsidize volume and make a dashboard look active. You cannot fake whether serious capital can enter and exit without moving the market. I'd rank prediction markets by OI, depth, spreads and resolution quality before headline volume.