Given that Simancas was created in the 16th century to house the documents issued by the central government bodies of the Spanish monarchy, it is only logical that its collections should reflect the administrative structure from the reign of the Catholic Monarchs (end of the 15th century) to the fall of the Ancien Régime (early 19th century). Over this long period two eras periods be distinguished: that of Austria (16th-17th) and that of the Bourbons (18th). The Simancas collections, consequently, are comprised of two large blocks: those belonging to the period of the House of Austria and those of the period of the Bourbons.
The government bodies at the time of the House of Austria were the Councils. There were as many Councils as territories of the Spanish monarchy (Council and Chamber of Castile, Aragon, the Indies, Italy, Flanders and Portugal) and as many general matters dependant on the latter (Council of State, of War, of the Treasury, of the Inquisition, of the Orders and of the Crusade). Thirteen Councils or bodies through whose hands all matters regarding the above mentioned territories and affairs passed. In Simancas, therefore, for the 16th and 17th centuries there are as many sections as there were Councils, with four exceptions: the Councils of the Orders, the Indies, Aragon and the Inquisition.
The government bodies at the time of the House of Bourbon were the Bureau Secretariats. The Bourbons created five government bodies: the State Bureau Secretariat, and the Secretariats for War, Treasury, Grace and Justice and Navy and the Indies. In Simancas, therefore, for the 18th century there are as many sections as there were Secretariats.
Besides the aforementioned sections, there are factitious sections that did not originate from an administrative body: Royal Patronage, and Maps, Plans and Drawings.