If anyone hasn't already read Chris Stucchio's explanation of market-making/HFT, this is well worth a read. https://www.chrisstucchio.com/blog/2012/hft_apology

If anyone hasn't already read Chris Stucchio's explanation of market-making/HFT, this is well worth a read. https://www.chrisstucchio.com/blog/2012/hft_apology.html https://www.chrisstucchio.com/blog/2012/hft_apology.html https://www.chrisstucchio.com/blog/2012/hft_apology2.html https://www.chrisstucchio.com/blog/2012/hft_apology2.html https://www.chrisstucchio.com/blog/2014/how_to_not_get_ripped_off_by_hft.html https://www.chrisstucchio.com/blog/2014/how_to_not_get_rippe... A basic summary is that market-makers add liquidity to the market and profit from their bid-ask spread, increasing the execution-speed and depth of the market. Many of the 'predatory' pricing strategies attributed to them by Lewis and others appear to be impossible when you try and write down the pseudo-code + order book that corresponds to the allegation.