Most existing information systems are developed to support stationary office workers. Rapid development in mobile communication provides a great potential for us to move from traditional office support to mobile work support. Despite this great potential, we still lack the theory to fully understand the nature of mobile work. Without this understanding, it will be difficult to develop efficient and effective support for mobile workers. In this paper we propose a theoretical mobile work support framework and use this framework to analyze four fundamental aspects of mobile work: mobile workers, mobile tasks, mobile context, and mobile technology. The key differences between office work support and mobile work are also highlighted.