High frequency trading dominates trading in financial markets. How it affects the low frequency trading, however, is still unclear. Using NASDAQ order book data, we investigate this question by categorizing orders as either high or low frequency, and examining several measures. We find that high frequency trading enhances liquidity by increasing the trade frequency and quantity of low frequency orders. High frequency trading also reduces the waiting time of low frequency limit orders and improves their likelihood of execution. Our results indicate that high frequency trading has a liquidity provision effect and improves the execution quality of low frequency orders.