"Using while moving" is the basic ability fieldwork users require of a mobile computer system. These users come from a wide range of backgrounds but have in common an extremely mobile and dynamic workplace. We identify four specific characteristics of this class of users: dynamic user configuration, limited attention capacity, high-speed interaction, and context dependency. A prototype is then presented that was designed to assist fieldworkers in data collection tasks and to explore the HCI design issues involved. The prototype was used in an extensive field trial by a group of ecologists observing giraffe behavior in Kenya. Following this trial improvements were made to the prototype interface which in turn was tested in a subsequent field trial with another group of ecologists. From this experience, we have formulated our resulting ideas about interface design for fieldworkers into two general principles: Minimal Attention User Interfaces (MAUIs) and context awareness. The MAUI seeks to minimize the attention, though not necessarily the number of interactions, required from the user in operating a device. Context awareness enables the mobile device to provide assistance based on a knowledge of its environment.