Medieval Hispanic sources give great importance to the operarius. This term has been translated in Spanish historical works as canónigo obrero, obrero mayor or fabriquero, whose principal function was to collect and administer the construction revenues. The operarii were a link between the master builders (magister operis) and the bishops, since they were acting in representation of the Chapter in matters related to the financing and administrative organization of the construction. The present paper aims to answer some questions concerning the profile of the operarius, such as his social condition (ecclesiastic-layman?), his competences (administrator or master builder?), as well as his cursus honorum in the monastic or cathedral communities. For that purpose, different cases studies in the Hispanic kingdoms are put forward: Ramon Lambard, lay master builder and operarius of la Seu d’Urgell cathedral; Berenguer d’Obició, operarius and prior of la Seu Vella of Lleida, or Pedro Carnaz (Zaragoza cathedral).