Hand-held mobile computers have the potential to become important communication tools for roaming users. As such, they will also become very personal. They will be used under a wide range of operating conditions, and tight user control will be enforced on issues like power consumption, consistency control, and trust management. Their ability to adapt will be the key to their success. In this paper we outline our notion and use of Quality of Service (QoS) to the design of adaptive software systems for mobile computers. They have been developed in the MobyDick and GDD projects. We do not emphasize on the provision of QoS guarantees. In stead, our notion of QoS is used to convey relevant and timely management information between service users and providers on the correct abstraction level. It structures adaptability management in the hand held machine, and it captures adaptability to changes both stemming from the hosting environment and user commands. As an example of how the architecture works, the importance of adaptivity of security services for personal companions are explained, and we show how our notion of QoS may realize adaptable security services.