Josep M. Millàs Vallicrosa’s correspondence reflects the breadth of his international scholarly network, particularly his relationship with George Sarton, founder of the journal Isis, with whom he exchanged 75 letters between 1929 and 1955 in various languages. This correspondence, crucial to the history of science, documents the genesis and dissemination of his work, as well as his involvement in international institutions and congresses. Letters to (and from) notable figures such as Willy Hartner and José María Albareda also illustrate the recognition and influence Millàs held in the institutionalization of the history of science both in Spain and internationally.