This article is a high level overview of the situation and doesn't get into some of the lower level technical issues with HFT firms. Just as one example: ISO (intermarket sweep orders) are routed from brokers to the different exchange venues. HFT firms have optimized their latency to the point they can actually see an ISO order on exchangeA and quickly cancel their orders on exchangeB, exchangeC before the customers order routes to the other exchanges from the broker. This effectively amounts to phantom liquidity since the size vanishes before the trade can fully execute across all venues. There are many technical games like this that are played, and its cat and mouse game with HFT and the SEC. It would serve investors well to have a single regulated exchange where all the liquidity is concentrated. The simple idea that market makers are valuable because they provide liquidity ignores the fact they play numerous technical games which are harmful to retail investors.