This paper presents general aspects of the water budget (including the latest available figures from its components), followed by some characteristics of the rainfall along the Paraiba do Sul watershed. The regional water budget theme is then discussed, taking into account the actual and future rainfall in the region. Finally, reflections and discussions regarding the water security theme (represented by the water transposition from the Paraiba do Sul watershed to the Cantareira´s system) are discussed, as the latter represents the water demand of São Paulo City. The future scenarios of the water availability are estimated using a climatic simulation model.