In pervasive computing environments, conditions are highly variable and resources are limited. In order to meet the needs of applications, systems must adapt dynamically to changing situations. Since adaptation at one system layer may be insufficient, cross-layer, or vertical approaches to adaptation may be needed. Moreover, adaptation in distributed systems often requires horizontal co-operation among hosts. This cooperation is not restricted to the source and destination(s) of a data stream, but might also include intermediate hosts in an overlay network or mobile ad hoc network. We refer to this combined capability as universal adaptation. We contend that the model defining interaction between adaptive middleware and the operating system is critical to realizing universal adaptation. We explore this hypothesis by evaluating the Kernel-Middleware eXchange (KMX), a specific model for cross-layer, cross-system adaptation. We present the KMX architecture and discuss its potential role in supporting universal adaptation in pervasive computing environments. We then describe a prototype implementation of KMX and show results of an experimental case study in which KMX is used to improve the quality of video streaming to mobile nodes in a hybrid wired-wireless network. Copyright 2004 ACM.