As EU citizen you are entitled to free move around the EU without having to fill in forms or specify the reasons for travelling. The only document you need is a valid identity card or passport.
The free movement of people within the European Union was established for the first time in the 1985 by the Schengen Agreement, originally signed only by France, Belgium, Luxemburg and the Netherlands, which created a “Schengen area” free of internal borders.
In order to match together liberty and safety, the Schengen Agreement envisaged a common policy concerning visa, right of asylum and external borders control, as well as customs and organized criminality. For this reasons, a complex “Schengen Information System” aims to share data about both people identity and researched objects’ descriptions.
On 1997, by the Treaty of Amsterdam, the Schengen Agreement has been placed into the EU context and progressively extended to all Member States, apart from Ireland and United Kingdom, which preferred to maintain their internal borders control, and, partially, also to the 10 State which entered EU in 2004. To date 29 Countries are part of the Schengen Area: the EU Member States (with exception of UK and Ireland), Iceland, Norway, Switzerland and Liechtenstein. Among the EU Member States, Cyprus, Bulgaria and Romania are still implementing the agreement, therefore controls remain at the borders.
Therefore, thanks to these measures you can cross internal EU borders without showing your documents. The only reasons which allow Member State to temporary control their borders are related to public order, safety or health matters.
Anyway, please notice that, although being part of the Schengen area, Member States continue having the right to control people’s identity within their borders, according to both national legislation and police powers. Therefore, it might be always useful bring with you a valid ID card and, when required, your permit of stay.