1570 Master of st. Luke's Guild, between 1575-1612 regularly 'Vinder' (member of the governors) and between 1586-1602 threetimes dean; 1570-1581 City painter: in the first years of employment as a city painter he made preparations for the procession of the Holy Blood. Presumably he also painted for the magistrate 'Ahasuerus' Feast', signed and dated. 1574 (Bruges, Groeninge-Museum). 'The Last Supper' by his teacher Pourbus (Bruges, Sacrament's Chapel) was damaged during the 1567-68 iconoclasm and must have been restored by Claessens. 1590-1593 he painted the Last Supper himself for the governors of St. Gilles with their portraits on the side wings; 1605 he painted the Allegory of Mars surrounded by the seven liberal arts (signed and dated 1605); In 1609, Bishop Rodoan of Bruges commissioned him to paint the triptych of the Deposition (center panel) and the portrait of Karel-Filips of Rodoan and the apostle Philipus, dated 1609 (Bruges, St.Salvator) (Saur 1998).