This paper examines the order submission strategies and supply of liquidity by highfrequency participants versus the remainder of participants in the limit order book. The results show that high-frequency participants submit orders at multiple prices in the limit order book, concentrated at or within the quote. This activity translates into the provision of liquidity on an on-going basis, which is robust to fast versus slow and volatile markets, together suggesting that high-frequency participants resolve temporal liquidity imbalances in the limit order book. The evidence is consistent with high-frequency trading (HFT) improving market liquidity, but there remain issues surrounding high-frequency participants' effect on market depth and the difficulty of trading of non-HFT participants.