Spatial reasoning is a diverse topic; what might different spatial tasks have in common? One task where substantial progress has been made is qualitative spatial reasoning about motion. Unlike qualitative dynamics, purely qualitative spatial representations have not proven fruitful. Instead, a diagrammatic representation appears to be necessary. This paper begins by outlining the Metric Diagram/Place Vocab. ulary (MD/PV) model of qualitative spatial reasoning, illustrating its power with via two example systems -- FZ0B, a system which reasoned about motion, and CLOCK, a system which analysed fixed-axis mechanisms. We believe this model is applicable beyond simply reasoning about motion. We suspect that (1) some form of metric diagram is a central unifying factor in all spatial reasoning tasks and (2) for human spatial reasoning, the metric diagram is part of, or at least grounded in, our perceptual apparatus. In this spirit, we identify three other kinds of spatial reasoning tasks as research frontiers where substantial progress might also be made, and pose six challenge problems to serve as milestones. The frontiers are (I) deriving system function from concrete structural descriptions (2) representing and reasoning about spatially distributed systems and (3) explicating the role visual perception and recognition in reasoning.