The canon law rules addressing 'Church Pollution' provide a long-standing example of social regulation. A survey of historical material, and secondary sources, identifies a sophisticated set of precepts that developed over centuries. This development included application to one of the most important events in medieval England: the murder of Thomas Becket. Perhaps more importantly, the regime was widely used and thus of great significance to the ordinary citizens of the Middle Ages. Though largely historical, more recent examples of employment can also be found. When viewed through a contemporary lens, there are some connections that can be made with modern concepts of 'pollution' and contemporary environmental law and policy, such as that relating to contaminated land. While the relationships should not be overplayed, that analysis suggests a social and cultural heritage that has been drawn upon, whether consciously or not. When attempting to view matters from the perspective of medieval society, so conceptualising the 'environment' to include consideration and protection of the spiritual environment, further associations can be found. The differences in focus for the regulatory endeavours reflect differing fears, values and priorities. They also identify how these factors influence our definition and regulation of 'pollution'. Scully 'The unmaking of a saint: Thomas Becket and the English Reformation' (2000) 86 Cath Hist Rev 579 at 582. Most scholars agree that the knights initially intended no more than arrest, the murder resulting, in part, from Becket's refusal to go quietly: N Vincent 'The murderers of Thomas Becket' in N Fyde and D Reitz (eds) Bischofsmord im Mittelalter (Göttingen: Vandenhoeck & Ruprecht, 2003) p 211. 2. The full horror of the sacrilege included profaning the church, in the (sacred) Christmas season, with the blood and murder of a priest, cutting off the crown of his head (which had been dedicated to God), and scraping the brain out to scatter with blood and bones all over the panel floor: