The objective of this paper is to establish a definition of deep aquifers, develop a methodological proposal to help select them as strategic groundwater reserves and develop a first inventory of deep aquifers in peninsular Spain, organized by hydrographic districts. The basic premises followed to consider a permeable formation as a deep aquifer are: water table depth greater than 300 m in free aquifers or the top of the permeable formation in the case of confined aquifers. A deep aquifer is also considered to be the lower horizon of very thick aquifers (from 300 m), from which the most superficial horizon is used. The definition of the potentially favorable areas and the initial characterization is based on a compilation and synthesis of geological and hydrogeological information, structured in 5 large blocks: location and identification, geology, hydrogeology, current status and bibliography. For its evaluation, a flowchart has been proposed with the basic lines of the evaluation process. The relevant aspects for suitability would be: stratigraphy and structural geology, prior knowledge of the conceptual model and water renewal, quality, viability of exploitation, effects, final knowledge, and management and governance. The final result allows the aquifer to be classified into three categories: strategic groundwater reserve, punctual reserve or non-viable reserve. The inventory carried out encompasses a total of 64 deep aquifers located in 13 basin districts. Finally, as example, three deep aquifers are presented: Amblés Valley, Madrid detritic aquifer and El Maestrazgo.