Brazil's 26 states and the Federal District (Distrito Federal) are divided conventionally into five regions: North (Norte), Northeast (Nordeste), Southeast (Sudeste), South (Sul), and Center-West (Centro-Oeste) - see fig. 4. In 1996 there were 5,581 municipalities (municípios), which have municipal governments. Many municipalities, which are comparable to United States counties, are in turn divided into districts (distritos), which do not have political or administrative autonomy. In 1995 there were 9,274 districts. All municipal and district seats, regardless of size, are considered officially to be urban. For purely statistical purposes, the municipalities were grouped in 1990 into 559 micro-regions, which in turn constituted 136 meso-regions. This grouping modified the previous micro-regional division established in 1968, a division that was used to present census data for 1970, 1975, 1980, and 1985.
Each of the five major regions has a distinct ecosystem. Administrative boundaries do not necessarily coincide with ecological boundaries, however. In addition to differences in physical environment, patterns of economic activity and population settlement vary widely among the regions. The principal ecological characteristics of each of the five major regions, as well as their principal socioeconomic and demographic features, are summarized below.