First, this thesis aims to find, classify and explain the different reasons why the inhabitants of Barcelona use Catalan, Castilian or both languages according to a given situation. On the other hand, based on the results, it is intended to better understand the situation in Catalonia and the Catalans in a sociolinguistic field. The focus lies on answering the question “Is there a language that is preferably used by citizens living in Barcelona?”. With the use of a quantitative method analysis this thesis will reveal if there is a determining variable that influences when choosing Catalan or Castilian? Diferent types of resources have been used to verify and have legitimacy in the arguments presented in the thesis. The historical moments survived by the Catalan inhabitants, the identity (culture and linguistic) that they managed to preserve, and the bilingualism that exists in Barcelona, are topics that will pervade through the entire thesis. The result of the study shows that the respondents generally prefer to use Castilian rather than Catalan, for the reason of the origins (parents or grandparents) from outside Catalonia.