Both the issues of “knowledge management” and “mobility” have received much attention recently. The interest in these issues is often motivated by the fact that work, in many organisations, has become “knowledge intensive” and “mobile.” However, so far these issues have been explored separately. The thesis is a collection of six papers that address these issues from an Informatics perspective. Informatics can be described as a theory and design oriented study of information technology use, an artificial science which focuses on the intertwined complexity of people and information technology as its subject matter. The scope of this thesis is delimited to the design of co-operative technologies. The overall research question is: how can we provide mobile workers with timely knowledge? Here, timely knowledge means knowledge that is relevant for the task at hand. The thesis contains empirical studies of mobile work, technologies for knowledge systems, and the design and validation of prototype systems. The studies mainly consist of observations of mobile service electricians and mobile news journalists. They show that current perspectives on knowledge management do not adequately accommodate mobile work. Furthermore, implications for design are derived. Technologies to capture and measure text and hyperlink data are developed and refined to be used in knowledge systems. The design implications and the technologies serve as base for the design of three knowledge systems. One of them has been validated in several workshops and under real working conditions of mobile news journalists. The final contribution is a generalised technological architecture, designed to be easily adapted to several mobile work settings and emerging mobile technologies. The architecture is derived from the summation of previous results and the first practical implementation is in the domain of mobile news journalism.