Threads play a key role in object-based middleware platforms. Implementers of such platforms can select either kernel or user-level threads, but neither of these options are ideal. In this paper we introduce Application Scheduler Contexts (ASCs) which flexibly combine both types of thread and thereby attempt to exploit the advantages of each. Multiple ASCs can co-exist, each with their own concurrency semantics and scheduling policy. ASCs also support quality of service (QoS) configurability, and define their own QoS schema. We show how ASCs can be efficiently implemented and how they can usefully be exploited in middleware environments. We also provide a quantitative evaluation that demonstrates the feasibility of the ASC concept in performance terms. Copyright (C) 2002 John Wiley Sons, Ltd.