The purpose of this article is to reconstruct and interpret the constitution of the Communist Youth of Chile in relation to the social, worker and student mobilizations of the 1930’s. Between the fall of the Ibáñez dictatorship in 1931 and the First National Congress of the Communist Youths in 1940, we observe a long process of national constitution of the "Jota". This process was marked disputes over alternatives to the crisis of the oligarchic order, the formation of the Popular Front and the complex processes of unity of social mobilization. We observe how the Communist Youths composed of workers, students, teachers, Mapuches and farmers forged their first political experiences. They were protagonists in various social movements, suffered and overcame repression, raised cells and defined the first attempts of a popular youth movement. We used party reports and bulletins, party press and police reports.El artículo analiza la formación de las Juventudes Comunistas de Chile entre la caída de la dictadura de Carlos Ibáñez del Campo en 1931 y el Primer Congreso Nacional en 1940, en un contexto de disputas sobre las alternativas de solución a la crisis del orden oligárquico, la conformación del Frente Popular y la compleja unidad de la movilización social. Observamos cómo las JJCC, compuestas por jóvenes obreros, estudiantes, profesores y mapuche, forjaron sus primeras experiencias políticas, protagonizaron los movimientos sociales, enfrentaron la represión y definieron los primeros intentos de un movimiento juvenil popular. Usamos documentos partidarios, prensa e informes policiales.