During the lifetime of an application running on a mobile host, resource constraints of the machine changes. Because of these constraints, system facilities that are provided in traditional computing environments such as dependence on static resources and continuous connectivity can not be relied on. New application development platforms are needed. MaROS is an object oriented client/server framework for application development on mobile hosts. It is designed as a set of library functions considering the constraints intrinsic to mobile computing and their impact on the design of applications. It offers advantages for the applications that require continuous access to resources even when running on a mobile host with certain limitations. Basically, the framework provides an adaptive client model through an object naming and creation scheme which supports object relocation, relocatable object maintenance, a recovery agent that brings the system to a proper state when a mobile host owning relocated objects properly shuts down --because of a battery run out-- and comes up again, and a communication infrastructure based on queued operations. In this paper, the components of the MaROS environment and their design principles are presented.