Aggressor

In every futures trade, there are two parties: a passive participant who posts a resting limit order, and an aggressive participant (the aggressor) who fills against that order using a market order or marketable limit order. The aggressor takes liquidity; the passive participant provides it.

Aggressor classification

Buy aggressor: a trader who submits a buy market order (or marketable limit) that executes against resting ask-side limit orders. This prints on the ask and is classified as buy volume.

Sell aggressor: a trader who submits a sell market order that executes against resting bid-side limit orders. This prints on the bid and is classified as sell volume.

This classification is how all delta calculations are derived. There is no ambiguity: every fill either hits the bid (sell aggressor) or lifts the ask (buy aggressor).

Why aggressor direction matters

The aggressor tells you who is impatient: who wanted a position badly enough to accept the current market price rather than waiting for price to come to them. Large or sustained aggressor activity in one direction indicates conviction and urgency.

Passive limit order players, by contrast, are willing to wait. They often represent larger, more patient participants (market makers, large institutions) who know their order size would move price if submitted aggressively.

Aggressor and price movement

Price moves when the aggressor’s volume exceeds what the passive side is willing to provide at the current level. When buy aggressor volume overwhelms the ask-side depth, price lifts to the next offer. When sell aggressor volume exhausts the bid-side depth, price drops to the next bid.

Understanding the balance of aggressor and passive activity: who is winning, and at what levels the passive side holds or gives way: is the core of order flow analysis.