General-purpose operating systems provide inadequate support for large-scale servers. Server applications lack sufficient control over scheduling and management of machine resources, which makes it difficult to enforce priority policies, and to provide robust and controlled service. For example, server applications cannot provide differentiated quality of service to requests from different clients. The root cause of these problems is a fundamental mismatch between the original design assumptions underlying the resource management mechanisms of current general-purpose operating systems, and the behavior of modern server applications. In particular, the notions of protection domain and resource principal coincide in the process abstraction of current operating systems. Moreover, these operating systems provide insufficient control to an application over the resources that are consumed inside the kernel on behalf of the application. These aspects of current operating systems prevent a server process that manages large numbers of network connections, for example, from properly allocating system resources among those connections. This dissertation addresses the lack of operating system support for fine-grained resource management in large-scale server systems. It starts by characterizing the nature of the mismatch between the design assumptions of current general-purpose operating systems, and the behavior of server applications. The traditional design of core operating system abstractions and APIs is reevaluated in the light of the requirements of server applications. This reevaluation leads to a set of novel operating system abstractions and APIs that serve to provide effective support for server applications.