The French artist probably moved to Rome in 1613, where he underwent the influence of Caravaggio. After a difficult period, he found favour with Cardinal Francesco Barberini from whom he obtained numerous commissions among which the Martyrdom of St Processo and St Martiniano, which gave him definitive fame. The painting, completed by 1629, was executed for an altar of the right transept of St Peter's Basilica. The references to the Martyrdom of St Erasmus, painted by Poussin for a nearby altar, are clear, so much so that the two works were considered from the beginning as being in competition with one another. Contemporaries acknowledged that the purely Caravaggio style painting (due to its realism and the particular use of light) of Valentin exceeded that of Poussin in naturalism, force, richness and harmony of colour.