Episciences is a platform to publish diamond open access overlay journals. It offers an alternative to the monolithic system of scientific publication dominated by private publishers. The ambition is to provide the scientific communities with the technical means to produce high quality journals, at an efficient cost, compliant by design with FAIR principles, by relying on open archives and repositories in general (arXiv ; Zenodo ; HAL ; CWI). The mechanism implemented is simple: the submission of an article to a journal is done by a prior deposit in an open archive. The article is freely accessible and therefore consultable by all, whether or not it is accepted by the journal.