Research in the Life Sciences increasingly involves the investigation of fast dynamic processes at the cellular and sub-cellular level. It requires tools to image complex systems with high temporal resolution in three-dimensional space. For this task we introduce a fast fluorescence line scanner with image acquisition speeds in excess of 100 frames per second at 512x512 pixels and with a more than 10- fold increased sensitivity compared to point scanning confocal systems. Since the system preserves the capability for optical sectioning of confocal systems it allows to observe processes in three dimensions. We describe the principle of operation, the optical characteristics of the microscope and cover several applications in particular from the field of developmental biology.