The Polysynodial System, Polysynodial Regime (Spanish: régimen polisinodial) or System of Councils was the way of organization of the composite monarchy ruled by the Catholic Monarchs and the Spanish Habsburgs, which entrusted the central administration in a group of collegiate bodies (councils) already existing or created ex novo. Most of the councils were formed by lawyers trained in academic study of Roman law. After its creation in 1521 (and 1526 revamp), the Council of State, chaired by the monarch and formed by the high nobility and clergy, became the supreme body of the monarchy. The polysynodial system met its demise in the early 18th century in the wake of the promulgation of the Nueva Planta decrees by the incoming Bourbon dynasty, which organized a system underpinned by Secretaries of State.

Its origin goes back to the Middle Ages in the consultative bodies of the crowns of Castile, Aragon and Navarre. The basic mechanism of operation was the elevation of a consultation to the monarch, who resolved according to their opinion.

The councils were of three types:

(Spanish: Consejo de Estado)
Established under Charles V, the Council of State did not have specific areas of focus or competences, nor territorial scopes determined by what was supraterritorial, which was the reason why the most important subjects and of foreign policy, as well as the subjects relative to the monarch and to the royal family. It was established in 1526 when Suleiman the Magnificent threatened Austria.

It was the only Council that did not have a president, since it was the monarch who assumed that function. His advisors were not specialists in legal matters but in international relations, like the Duke of Alba or Granvela. The councilors were, therefore, members of the high nobility and the high clergy. Its mission was to advise the monarch on foreign policy and had control of the embassies of Rome, Vienna, Venice, Genoa, and the major powers of Europe: France, England and Portugal.

Unlike the Council of Castile, in which the monarch listened to the councilors and executed the conclusions they presented, in the Council of State it was the monarch himself who exposed the points to be discussed, listened to the counselors and, subsequently, made decisions. Related to the Council of State was the Council of War, they had the same members, except that the Council of War had specific advisers, and the matters they were in charge of everything related to the armies, equipment, appointments, war planning and last judicial instance.

(Spanish: Consejo de Hacienda)
Established under Charles V, Holy Roman Emperor, the creation of the Council of Finance (Hacienda) in 1523 was a remarkable rationalization of the Castilian treasury, which until then had two accounts that were continually facing each other (the Major Account, responsible for the collection of taxes and their administration, and Accounts, which was responsible for intervening and verifying the accounts of the previous one).

This council consisted of three counselors, normally university graduates with experience in the bureaucracy of the Court and four assistants (Treasurer, finance clerk, accountant and secretary), to decide on matters of the Royal Treasury, were adopted by the full Council or by its three most important courts: the Court of Millions, the Court of Oidores and the Accounts Accountant. Their competences included collecting taxes, administrating them and ensuring that their collection was fulfilled; Executing expenses, proposing new funding sources, proposing budgets and requesting reports from accountants (something like the current accountants) of the other Councils. This Council was characterized by the continuous friction with the other Councils, aggravated by the fact that the members of the Finance Council were rarely of important families, unlike the rest of the Councils.