Iwi () are the largest everyday social units in Māori society. In pre-European times, iwi was synonymous with nation; it described fully the people to whom a person belonged and owed allegiance. With the development of the country now called New Zealand, a much bigger social unit, the meaning became analogous to that of tribe or clan.
Iwi groups can trace their ancestry to the original Māori settlers that arrived from Hawaiiki, at least according to tradition. Māori who know their iwi connections typically value them highly and place great pride in knowing their genealogy. Their origin is among the first things they mention when introducing themselves.
Many iwi cluster into super-groups based on genealogical tradition, known as waka (literally: "canoes", i.e. the original migratory canoes). Each iwi can be divided into a number of hapu ("sub-tribes"). (For example, the Ngāti Whātua iwi consists of the hapu: Te Uri O Hau, Te Roroa, Te Taou, and Ngāti Whātua ki Orakei.)
Despite migration within New Zealand and intermarriage with non-Māori over a couple of centuries, most iwi groups still exist and have significant political power, which they exercise to recover land and other assets taken from them over the last 150 years. A notable example of this is the recent settlement between the New Zealand Government and the Ngāi Tahu, compensating that iwi for various losses of the rights that were guaranteed in the Treaty of Waitangi of 1840. Iwi affairs have a very real impact on New Zealand politics and society. A current claim by some iwi that they own the seabed and foreshore in their areas has polarised public opinion (see New Zealand foreshore and seabed controversy).
In the 2001 census, 32.6 per cent of the 604,110 people who claimed Māori ancestry did not know their iwi, or only stated a general geographical region or merely gave a canoe name. It seems that the number who "don’t know" has remained relatively constant over the last three censuses, despite measures such as the "Iwi Helpline".
They are typically urban bred, and probably identify with European culture to a much larger degree than traditional Māori, and often feel that a non-iwi group best represents their needs. How the traditional iwi groups respond to this remains to be seen. (As yet, some appear dismissive of these notions.) Notably, one such group has been created believing that Urban Māori are not getting their fair share of "treaty settlements" between the Māori people and the New Zealand government.
Note that each iwi has a generally recognised territory (rohe), but many of these overlap, sometimes completely.* This has added a layer of complication to the long-running discussions and court cases about how to resolve historical Treaty claims. The length of coastline was one factor in the final (2004) legislation to allocate fishing rights in settlement of commercial fisheries claims.