This paper presents the Merlin network management framework. With Merlin, network administrators express functionality such as accounting, bandwidth provisioning, and traffic filtering in a high-level policy language, and use automated tools and mechanisms to implement them. The framework includes: (i) a declarative language for specifying policies, (ii) infrastructure for distributing, refining, and coordinating enforcement of policies, and (iii) a run-time monitor that inspects incoming and outgoing traffic on end hosts. We describe Merlin's policy language and enforcement infrastructure, illustrate the use of Merlin on case studies, and present experimental results demonstrating that Merlin is more efficient and scalable than equivalent implementations based on programmable switches and centralized middleboxes. Overall, Merlin simplifies the task of network administration by providing high-level abstractions and tools for specifying and enforcing rich network policies.