To properly understand slippage, let's use the following example: Say, you (as a trader) wanted to purchase 20,000 shares of SPY right now. The problem here is that the current ASK price of $151.08 only contains 3900 shares being offered for sale, but you want to purchase 20,000 shares. If you need to purchase those shares now, then you must use a market order and you will incur slippage by doing so. Using a market order to purchase your 20,000 shares would yield the following executions (assuming no hidden orders in the market depth):