It is located in the southeastern part of the province of Segovia (Castilla y León, Spain), within the municipality of Real Sitio de San Ildefonso, and borders the Community of Madrid.

The valley is covered by an extensive pine forest known as el Pinar, the largest in the mountains and one of the best preserved in Spain.

The explanation for this excellent state of conservation lies in its origin as a hunting reserve of the Spanish monarchy and the subsequent sustainable use by the timber industry.

[2] The timber exploitation is linked to the sawmill, the Real Aserrío de Valsaín, which began its activity under the patronage of the Royalty in the year 1884.

At the beginning, the mass of young pines is very dense and decreases by natural process and by human intervention—through performing different cleanings and cuts until reaching the convenient density for an optimal development of the tree.

In these mountains, there is a wide diversity of natural habitats, ranging from the oak groves at the bottom of the valley to the grasslands on the summits, passing through the pine forests that occupy most of the territory, the rocky outcrops, the peat bogs, the banks of rivers and streams, the holly oak groves, etc.

[4] Valsain pine forest contain the largest number of protection figures that a natural area in Spain can contemplate, since 3,326 hectares of them belong to the Sierra de Guadarrama National Park, the remaining 7,011 hectares belong to a figure of their own called Área de Especial Protección del Parque Nacional (English: Special Protection Area of the National Park), created ad hoc to be able to make commercial forest management compatible.

These are Special Protection Area (SPA) and Site of Community Importance (SCI) due to the natural values it treasures.