The Courts of First Instance and Preliminary Investigation are unipersonal courts that exercise jurisdictional functions of both civil and penal nature within the territory of their authority, known as the judicial district. The judicial district takes its name from the municipality in which the court is situated.

In civil matters they offer the first answer to citizens about the conflicts that may arise before them. Their decisions may be appealed against before the Provincial Court of the province in which they are situated.

In penal matters they are entrusted with holding Trials of minor offences, as well as carrying out the preliminary investigations of offences, which will be tried by Criminal Courts, or, where applicable, Provincial Courts.

They are the judicial bodies which are closest to the citizens – as much for their location in the administrative centre of the judicial districts as for their competence, which requires, in the majority of cases, the presence in their premises of those involved for the taking of evidence.