The essay analyzes the relationship between collective agreements and individual labour contracts. After underlining that the collective agreements, with some specific clauses, expressly establish that their norms are mandatory, the author maintains that in many cases the law qualifies the collective agreement as compulsory and not modifiable «in peius» by the individual contract. In this situation, the clause of an individual contract which does not respect what is established by the collective agreement (introducing a worse discipline) is void according to articles 1418 and 1419 of the Italian Civil Code and will be automatically replaced by the regulation of this agreement. After criticising some of the theories which try to justify the prevalence of the collective agreement over the individual contract, the author maintains that this prevalence can be also reaffirmed in the cases in which the law does not qualify the collective agreements as mandatory. In fact, this characteristic can be considered an expressed or unexpressed «general principle» of our legal system, which can always be applied by the judge.
Keywords: Collective agreement; Individual labour contract; Relationship; Invalidity; Law imperative norm; General principle of the legal system.