An analysis of the observed precipitation for the last 22 years (1989–2010) over Sierra de Guadarrama (centre of Iberian Central System) has been performed. Since this area has received less attention compared with other mountain ranges in Europe and the rest of the world, an exhaustive compilation of literature on precipitation main characteristics on Central Iberia (SW Europe) has been done. The analysis, based on both rain gauges and reanalysis, is focused on the search of the atmospheric mechanisms and moisture sources that lead to precipitation. Also, emphasis has been made in the role played by orography conditioning, the complex spatial patterns observed in the region. This work shows how it behaves as an orographic island that rises over an extensive plateau with a marked Atlantic forcing despite the distance these masses have to travel inlands, with mostly of wet days due to advection of moisture from the ocean and small amount of rain due to local sources of humidity and convective precipitation. It also shows the great enhancement of precipitation caused by the range due to orographic precipitation and the big differences found at the downwind and upwind side of the mountain leading to marked spatial patterns seasonal dependent, complex vertical gradients and high wind direction dependency. Finally, statistically significant mean conditions of variables related with orographic precipitation are given in order to be used as boundary conditions for orographic precipitation modelling exercises. This works gives a climatic framework for future precipitation assessments that could be conducted combining measurements and such models.