In terms of composition, The Blinding of Samson is regarded as Rembrandt’s most ambitious history painting of the 1630s. As past technical studies have shown, the creation of this highly dramatic image involved multiple reworkings. This article discusses new findings on the work’s genesis, as revealed by micro-X-ray fluorescence analysis (micro-XRF). Rembrandt used underdrawing and underpainting of various colours to work out the composition; he discarded entire figures and he continued reworking the painting even into the final layers of paint. Not only do these findings make clear that Rembrandt used the large format as a testing ground and wrestled with the composition, but they also clarify this painting’s relation to other contemporary versions of the subject, allowing conclusions to be drawn about Rembrandt’s inclusion of his own and Saskia’s facial features in the work.