In contrast to scholars and signers in the nineteenth century, William Stokoe conceived of American Sign Language (ASL) as a unique linguistic tradition with roots in nineteenth-century <i>langue des signes française</i>, a conception that is apparent in his earliest scholarship on ASL. Stokoe thus contributed to the theoretical foundations upon which the field of sign language historical linguistics would later develop. This review focuses on the development of sign language historical linguistics since Stokoe, including the field's significant progress and the theoretical and methodological problems that it still faces. The review examines the field's development through the lens of two related problems pertaining to how we understand sign language relationships and to our understanding of cognacy, as the term pertains to signs. It is suggested that the theoretical notions underlying these terms do not straightforwardly map onto the historical development of many sign languages. Recent approaches in sign language historical linguistics are highlighted and future directions for research are suggested to address the problems discussed in this review. While these relationships are not considered genetic, the theory does not make clear how to positively define the relationship; witness the unwieldy term “genetic nonrelatedness” in Thomason ( 2002 , p. 105). The Diversification of Sign Languages via Processes Other Than Linguistic Descent Sign languages have been grouped into language families based on a variety of types of evidence, including extra-linguistic evidence, such as historical connections among deaf educators and educational institutions, linguistic evidence, and a combination of both types of evidence (see Fischer, 2015 ; Reagan, 2021 for recent discussions). If sign change cannot be regular, or if the historical development of many sign languages has not resulted in regular correspondences across languages that are thought to be related, then it is not possible to use the Comparative Method As with Woodward's approach, these authors take an expansive view of cognacy: “[t]o determine a cognate relationship, researchers make an informed decision with the help of either folk etymology or additional scientific excavation for evidence of historical relatedness between the current LSF form and the modern ASL form” (Supalla and Clark 2015 , p. 90). The archaeological approach to identifying cognates does not seek to differentiate vocabulary in ASL that has been inherited via linguistic descent from vocabulary that has entered the language via other processes. Supalla and Clark ( 2015 , p. There are clear parallels in traditional historical linguistics to the distinction between homology and analogy. Greenberg's ( 1957 ) four causes of similarity differentiate between two causes that are thought to be historical—namely inheritance and borrowing—and two others that are considered nonhistorical—chance and sound symbolism. According to List ( 2016 ), however, there is no broadly accepted theoretical notion in historical linguistics that corresponds to the notion of homology. Theories in historical linguistics are certainly concerned with processes of language diversification via linguistic descent; they are also concerned with borrowing. But, historical linguists do not commonly make reference to an overarching term to describe both inherited and borrowed features. In parallel to the concept of homology, List ( 2016 ) proposed the term etymological relation to encompass the historical relations of cognacy and borrowing (see also “sign language etymology,” Supalla and Clark, 2015 ). List's invocation of etymology seems appropriate as a parallel to homology because the concept has a long history in linguistics with precisely this meaning. Mailhammer ( 2015 , p. 424) defined an etymology as “a historical account of the origin and the subsequent historical development of a linguistic item.” He distinguished between “internal” and “external,” or “contact,” etymologies. An internal etymology is one that describes the history of an inherited linguistic feature, whereas a contact etymology implicates borrowing events, or horizontal transmission. Mailhammer ( 2015 , p. 432–433) pointed out that “the etymology of a linguistic item can comprise one or more cases of horizontal transmission” and that “a contact etymology necessarily combines internal and external etymologies, vertical and horizontal transmission.” Thus, in parallel to homologous biological traits, the linguist may speak of etymologically-related words, the histories of which connect at a shared common etymon. List's notion of the etymological relation accurately captures the type of historical relation that the less precise notion of influence is intended to invoke in the theory of sign language relationships described in Section Theoretical adaptation of the cognate. Characterizing the historical relations of many signs as etymological directly acknowledges the methodological problems facing sign language historical linguistics—in particular the current inability to identify cognates using the Comparative Method. In contrast to previous theories about sign language relationships, the notion of etymology maintains important theoretical distinctions between vertical and horizontal pathways of descent in the histories of signs and linguistic features. A contact etymology, per Mailhammer, is flexible enough to incorporate instances of both vertical and horizontal transmission in the history of a sign, without committing historical linguists of sign languages to any conclusions about the genetic language relationships of the sign languages being compared. Conclusion The two subfields of historical linguistics—namely, those focusing on spoken and signed languages—have rarely engaged one another, despite the relevance of both subfields to an overarching theory of language change. Why have they so rarely engaged with each other?