An order book is the queue of buy and sell orders that have been submitted but not yet executed. It shows how much demand and supply is available at different prices. Prices move when aggressive orders consume the liquidity resting near the best bid or best ask.
How it works
The highest resting buy price is the best bid and the lowest resting sell price is the best ask. An aggressive buy order trades against the ask side first; if there is not enough size at that price, the remaining quantity must trade at higher levels. Aggressive selling works in the opposite direction.
The quote at the top of the book is only part of the picture. Market depth measures how much size is available across nearby price levels. Two markets can show the same best bid and ask but react very differently to a large order if one has much less depth behind those quotes.
Why it matters
Thinking in terms of an order book is more useful than saying that a price rose because there were 'more buyers than sellers.' Every executed share has both sides. What changes is urgency: one side becomes willing to cross the spread and consume the orders already waiting.
This matters most around macro shocks, earnings, and policy decisions. Liquidity providers can reduce or cancel quotes when uncertainty rises, so a market that looked deep a few minutes earlier can suddenly require much larger price moves to absorb the same trade size.
A simple market example
During the 2026 Middle East conflict, U.S. markets offered a recent example of liquidity changing under stress. The Federal Reserve's March FOMC minutes said broad equity prices fell about 5% over the intermeeting period and Treasury-market liquidity diminished as yield volatility increased. The Fed's July 2026 Monetary Policy Report again noted that Treasury liquidity had deteriorated during the heightened volatility following the conflict. The lesson is not that an order book 'caused' the macro selloff. It is that once uncertainty rises, fewer orders may remain close to the current price, so incoming buy or sell orders can consume several levels of the book much faster than they would in normal conditions.
Common mistakes
Treating a large displayed order as guaranteed demand or supply. Resting orders can be changed or canceled before they trade.
Assuming total buy orders versus total sell orders predicts the next move. The next trade depends on executable liquidity near the best prices and which side is willing to cross the spread.
Frequently asked questions
What is the difference between an order book and trade history?
The order book shows orders still waiting to trade. Trade history records transactions that have already occurred.
Why can market depth disappear around news?
When uncertainty and adverse-selection risk rise, liquidity providers may quote less size or pull orders until prices stabilize.
Does a deeper order book always mean lower risk?
No. More depth usually helps absorb trades, but displayed liquidity can disappear quickly during stressed markets.
Educational content only. Definitions describe common market usage and may vary by jurisdiction, instrument, or institution.