An Impi was a Zulu warband . The first impis were formed by Zulu king Shaka, who was then only the exiled illegitimate son of king Senzangakona, but already showing much prowess as a general in the army of Mthethwa king Dingiswayo in the Mthethwa-Ndwandwe war in the early 1810s.

Impi warriors were raised from the age as low as of six, joining the army as udibi porters at first, being enrolled into same-age groups (intanga) and being formally formed into regiments (ibutho) after their 20th birthday. They were trained to outrun a horse, cover about 80 km (50 miles) a day in foot and hide and stalk in the underbrush. They swore loyalty to the king of the Zulu tribe. They were not allowed to marry before they had proved their courage in some way.

Every impi was a thousand warriors strong and originally contained warriors from the same intanga (this practice later changed as casualties suffered by the regiments made reinforcements necessary). Each impi had its own colors in colored shields, headdress and other ornaments. An impi was also accompanied by udibi, young boys who carried implements like cooking pots and sleeping mats and on occasion acted as scouts. Shaka insisted that troops wear no shoes—they could run faster and were not disabled by the loss of their sandals. Training for this was to stamp thorns into the ground with bare feet.

In wartime, warriors painted their upper bodies and faces with chalk and red ochre. Their weapons were short stabbing spears (Iklwa ), cudgels (knobkerrie) and leather shields. The iklwa with its long (c. 25cm) tip was an invention of Shaka that superceded the older thrown assegai. It could theoretically be used both in melee and as a thrown weapon, but warriors were forbidden from throwing it, which would disarm them and give their opponents something to throw back (anyone who did not have his spear after a battle would be killed). At the time of Zulu War, king Cetshwayo also equipped them with muskets and they also used rifles captured from the British.

Shaka used impis with a modified circling tactic; Impi troops would divide into four groups. The main group (isifuba, 'chest') would face the enemy, two wings (izimpondo, 'horns') on two sides of the enemy and then force them towards the center. The fourth party (usually the veterans) remained as a reserve. They travelled light, and carried their own food or foraged along the way. Thus tactics against their enemies (other African tribes, the Boers, and the British) were surprise and overwhelming force, rather than siege or long campaigns. During the Anglo-Zulu War, British commander Lord Chelmsford complained that they did not 'fight fair'.

Against the Ndwandwe, numerically superior northern neighbours who invaded Zulu territory to suppress them, Shaka played hide-and-seek games, while laying waste to the land to prevent foraging. Shaka waited and only attacked when the Ndwandwe were divided or exhausted.

Impi were also famous for their custom 'washing of spears (in their enemy's blood)' in which they cut open the belly of killed (and allegedly sometimes still living) opponents. Supposedly this meant the release of the opponent's spirit so it could not haunt the killer.

Rudyard Kipling refers to them in his poem "Fuzzy-Wuzzy":

We took our chanst among the Khyber 'ills,

The Boers knocked us silly at a mile,

The Burman give us Irriwady Chills,

'An a Zulu Impi dished us up in style.