In recent decades, US equity markets have changed from predominantly manual markets with limited competition to highly automated and competitive markets. These changes occurred earlier for NASDAQ stocks (primarily between 1994 and 2004) and later for NYSE-listed stocks (mostly following Reg NMS and the 2006 introduction of the NYSE hybrid market). This paper surveys the evidence of how these changes impacted market quality and shows that overall market quality has improved significantly, including bid-ask spreads, liquidity, and transitory price impacts (measured by short-term variance ratios). The greater improvement in market quality for NYSE-listed stocks relative to NASDAQ stocks beginning in 2006 suggests causal links between the staggered market structure changes and market quality. Using proprietary data sets, provided by two exchanges, that identify the activity of high frequency trading firms, studies show these firms contributed directly to narrowing bid-ask spreads, increasing liquidity, and reducing intraday transitory pricing errors and intraday volatility.