Employment Contract Laws are under the Labour Legislation. All details and information related to the contract (information about the elements, main conditions related to the work performance, etc.) are regulated by the Royal Decree 1659/1998. (Ministerio de la Presidencia y para las Administraciones Territoriales)
The employment contract and the requirements to work in Spain for people that do not own the Spanish nationality varies depending on the country of origin. However, all citizens from the European Union Member States, Iceland, Liechtenstein, Norway and Sweden, can benefit from the same requirements.
All formalities ought to be done if the person is going to stay more than three months in Spain; the person must personally ask to be inscribed in the Central Registry of Foreigners in the Foreign Office (or police station) of the region the person is going to live. That is the only requirement (and only if the person is going to stay for more than 3 months) to stay and/or work in Spain. The foreign person from the mentioned countries will enjoy and be subject of the same laws and norms as the Spanish nationals. (Universitat de Barcelona)