The subject of this chapter is Grágás, the compilation of the laws of Iceland in the Commonwealth period. The chapter begins by outlining the court structure of Iceland and the fundamentals of legal procedure, briefly discussing the importance of law to the conversion narrative in Íslendingabók and its account of the first decision to put Iceland’s laws into writing. It describes the distinctive concepts and customs which underlie the legal system of medieval Iceland, looking at the role of the búi (neighbour) in legal procedure, and explaining the key concepts of helgi (the right of inviolability), grið (domicile, or household attachment), vígt (the right to kill or to avenge a wrong with impunity), and the problem of dealing with ómagar (dependants). The chapter argues that the laws and sagas are often mutually informing and demonstrates how fundamental an understanding of law is to the interpretation of the Íslendingasögur. It gives numerous examples of how the laws can be used to help elucidate the sagas, and uses the sagas to reveal the importance of law and legal knowledge in medieval Icelandic society.