The Semantic Firewall project deals with the enforcement of network security policies between trust domains in the presence of dynamically changing and unpredictable Grid communication needs. The problem is that whilst traditional static policies allow the type of access mechanisms required by Grid applications, the same mechansims can be exploited by crackers for malicious purposes, so firewall policies cannot remain static for long. By combining conventional Grid security with semantic reasoning methods, we aim to provide dynamic, adaptive network security that allows legitimate access but still prevents unauthorised access.