Since the reading of their father's will, Robert and Thomas le Parker had been arguing over a piece of land which their father left to Thomas. Robert decided to force Thomas into a settlement and on the day of the murder had taken two of his kinsmen to Thomas's house. Abducting Thomas to his own house, Robert locked the door and began to threaten Thomas with a knife. Thomas tried to escape but, finding the door locked, he seized the knife from Robert and killed his brother in self-defense. When discussing the involvement of family in criminal activities, it seems appropriate to begin with the oldest crime in our tradition — that of brother killing brother, Cain killing Abel. But despite the sensational qualities of this type of dispute, normally the families who appear in medieval criminal records were cooperating in felonious activities rather than directing their wrath toward each other. More typical is the rather pedestrian case of Simon Tut of Wappenham and his son Richard, who were convicted for stealing sheep in Bodecote in Northamptonshire. This paper will examine several quantitative indices which will shed light on both intrafamilial crime and that involving family cooperation: the frequency of familial as compared with non-familial crimes; the specific types of crimes involving family and the degree of kinship between family members appearing in criminal courts. Finally, the essay will suggest some general conclusions about tensions in rural society and within the medieval peasant family which contributed to family involvement in crime.