R ecent advances in distributed, mobile, and ubiquitous systems demand new computing environments characterized by a high degree of dynamism. Variations in resource availability, network connectivity, and hardware and software platforms influence the performance of the related user applications. The expected growth of ubiquitous computing over the next five years will further alter the nature of the computational infrastructure, bringing a plethora of small devices and requiring customized protocols and policies to fulfill users' evolving quality of service (QoS) requirements.During the past 10 years, software developers created middleware technology to facilitate development of software systems, most notably distributed and Internet-based, to support activities as diverse as scientific computation, information discovery and dissemination, and e-commerce. Middleware mediates interaction between the application and the operating system (hence its name). Related technologies, including the Object Management Group's CORBA, Sun Microsystems' Java-based J2EE, and Microsoft's .NET, hide from the pro-grammer the complicated details of network communication, remote method invocation, naming, and service instantiation, easing construction of complex distributed systems. CORBA, or Common Object Request Broker Architecture, and Java also hide the differences in the underlying software and hardware platforms, increasing portability and facilitating maintenance, as new versions of operating systems are released. Despite aiding development of distributed applications, conventional middleware technology lacks support for the dynamic aspects of the new computational infrastructure. Next-generation applications require middleware that can be adapted to changes in the environment and customized to fit into devices, from PDAs and sensors to powerful desktops and multicomputers [1,5]. Here, we reflect on how two independent research projects might influence the evolution of next-generation middleware. The reflective middleware model is a principled and efficient way of dealing with highly dynamic environments yet supports development of flexible and adaptive systems and applications.It's flexible and reconfigurable yet simple for programmers to use, notably for building dynamic distributed applications operating on the Net.

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