An ambush is a surprise attack carried out by people lying in wait in a concealed position. The concealed position itself or the concealed person(s) may also be called an "ambush". Ambushes as a basic fighting tactic of soldiers or of criminals have been used consistently throughout history, from ancient to modern warfare. The term "ambush" is also used in animal behavior studies, journalism, and marketing to describe methods of approach and strategy.

In the 20th century, a military ambush might involve thousands of soldiers on a large scale, such as at a choke point like a mountain pass. Conversely, it could involve a small irregular band or insurgent group attacking a regular armed-force patrol. Theoretically, a single well-armed, and concealed soldier could ambush other troops in a surprise attack.

In recent centuries, a military ambush can involve the exclusive or combined use of improvised explosive devices (IED). This allows attackers to hit enemy convoys or patrols while minimizing the risk of being exposed to return fire.

The use of ambush tactics by early people dates as far back as two million years when anthropologists have recently suggested that ambush techniques were used to hunt large game.

One example from ancient times is the Battle of the Trebia River. Hannibal encamped within striking distance of the Romans with the Trebia River between them, and placed a strong force of cavalry and infantry in concealment, near the battle zone. He had noticed, says Polybius, a "place between the two camps, flat indeed and treeless, but well adapted for an ambuscade, as it was traversed by a water-course with steep banks, densely overgrown with brambles and other thorny plants, and here he proposed to lay a stratagem to surprise the enemy". When the Roman infantry became entangled in combat with his army, the hidden ambush force attacked the Roman infantry in the rear. The result was slaughter and defeat for the Romans. Nevertheless, the battle also displays the effects of good tactical discipline on the part of the ambushed force. Although most of the legions were lost, about 10,000 Romans cut their way through to safety, maintaining unit cohesion. This ability to maintain discipline and break out or maneuver away from a kill zone is a hallmark of good troops and training in any ambush situation.

Ambushes were widely used by the Lusitanians, in particular by their chieftain Viriathus. Their usual tactic, called concursare, involved repeatedly charging and retreating, forcing the enemy to eventually give them chase, to set up ambushes in difficult terrain where allied forces would be awaiting. In his first victory, he eluded the siege of Roman praetor Gaius Vetilius and attracted him to a narrow pass next to the Barbesuda river, where he destroyed his army and killed the praetor. Viriathus's ability to turn chases into ambushes would grant him victories over a number of Roman generals.

Another Lusitanian ambush was performed by Curius and Apuleius on Roman general Quintus Fabius Maximus Servilianus, who led a numerically superior army complete with war elephants and Numidian cavalry. The ambush allowed Curius and Apuleius to steal Servilianus's loot train. However, a tactic error in their retreat led to the Romans retaking the train and putting the Lusitanians to flight. Viriathus later defeated Servilianus with a surprise attack.^{[page needed]}

Germanic war chief Arminius sprung an ambush against the Romans at Battle of the Teutoburg Forest. This particular ambush was to affect the course of Western history. The Germanic forces demonstrated several principles needed for a successful ambush. They took cover in difficult forested terrain, allowing the warriors time and space to mass without detection. They had the element of surprise, and this was also aided by the defection of Arminius from Roman ranks prior to the battle. They sprang the attack when the Romans were most vulnerable; when they had left their fortified camp, and were on the march in a pounding rainstorm.