||The first part of this two-part paper presents a comparative history of paranoia querulans, also known as litigants’ delusion, in German-speaking countries and France from the nineteenth century onwards. We first focus on two classic literary works which describe litigious behaviours that were later pathologized, then give an insight into the history of Querulantenwahn (litigants’ delusion), a term coined in 1857 by Johann Ludwig Casper and adopted by German-speaking psychiatrists and forensic experts. The last section is devoted to its French equivalent, the delusion of the litigious persecuted-persecutors. We show how this category, widely popular among French fin-de-siècle alienists, was replaced by another: the delusion of revendication (litigious subtype). The history of the vexatious litigants in the English-speaking world will be explored in the Part 2.