. Retail customer orders are usually filled by wholesale market makers who prefer to pay the brokerages to trade against these orders, rather than take liquidity from an exchange because it makes economic sense to them. Secondly, dark pools are able to provide price improvements to clients by executing trades at the mid-point of the bid-ask spread on lit exchanges, or providing marginal price improvement over the national best bid and offer (NBBO). Thirdly, market impact costs are significant for large block executions. Dark pools minimize market impact and information leakages that would otherwise put large block traders, who are usually pension funds or asset managers, at a disadvantage.