In the 1990s, exchange-traded funds (ETFs) were a novel idea. Many saw them simply as a new wrapper for traditional assets – a convenient repackaging of mutual funds. In reality, ETFs triggered a market structure revolution. By introducing creation/redemption mechanisms and arbitrage-driven liquidity, ETFs fundamentally changed how markets functioned and how investors accessed assets. ETFs blurred the line between primary and secondary markets and turned arbitrage into the mechanism for holding the system together.