Parliamentary Groups are the main actors of parliamentary life. They fix speaking times in debates and are legitimized to table almost all regulatory initiatives, except questions and requests for reports, whose initiative belongs to Members and amendments to sections, which they are both entitled to table. Likewise, legislative proposals may be tabled by Parliamentary Groups or by a minimum of fifteen Members.

They are set up in each Chamber at the beginning of each Legislative Term.

They may be set up by a minimum of fifteen Members of Congress, but they can also be formed by Members from one or more political parties which, though not attaining the said minimum, have secured no fewer than five seats and at least fifteen per cent of the votes cast in the constituencies where they have put forward a list of candidates or five per cent of the votes in the whole country.

 In no case may a separate group be constituted by Members of the House belonging to the same party, nor may a separate group be set up by Members who at the time of the general election belonged to political parties that did not oppose one another before the electorate.