The Biosphere Reserve includes Pyrenean valleys and siliceous (Northwest) and limestone (South and East) mountains, with peaks reaching 3,355 m (Monte Perdido). Landforms have been modelled by glaciations forming deep valleys with a U profile, circuses and lakes excavated by the ice (locally known as ibones). The limestone massifs also contain labyrinths of caves and galleries opened up by the water (Karst) the most famous being the sinkholes of the Escuaín canyon, considered to be among the deepest in Europe.

The climate is variable in function of the altitude and orientation, oscillating between the continental sub- Mediterranean zones in the lower parts and Boreal-Alpine climate in the highlands. The vegetation is distributed in altitudinal layers ranging from Oak groves to mosses and lichens in the highest areas near the perpetual snows. The intermediary layers are covered with a succession of Oak, Beech, Fir and European Black Pine, mixed with Birch, European Yew, Aspen, Ash and Willow trees. Plant diversity is very high and over 2,000 species have been recorded.

The fauna is interesting with species such as the Chamois and the Marmot which inhabit the Alpine grasslands or the Pyrenean Desman that inhabits mountain streams and, among the birdlife, the rare Lammergeyer, or Bearded Vulture, the Rock Ptarmigan, or in the remotest forests, the Western Capercaillie, the White-backed Woodpecker and the Black Woodpecker are noteworthy.

Highland stock-raising, essential for the stability of Alpine grasslands, and rural and nature tourism.