Although sign languages are crucial for research on the human linguistic capacity, argument structure in these languages is rarely addressed in theoretical and typological studies. This article provides an overview of existing research on argument structure and argument structure alternations in sign languages. It demonstrates that there are many fundamental similarities between the two modalities in the domain of argument structure, such as the basic valency patterns and the semantic basis of argument structure. At the same time, modality effects, such as iconicity, simultaneity, and the use of space, play an important role in argument structure realization. Finally, the article discusses how emerging sign languages present an opportunity to observe and study the emergence of argument structure. Expected final online publication date for the Annual Review of Linguistics, Volume 8 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.