 This entry, and its companion on  classical theories,  have been concerned with tracing the history and philosophy of ‘absolute’ and ‘relative’ theories of space and motion. Along the way we have been at pains to introduce some clear terminology for various different concepts (e.g., ‘true’ motion, ‘substantivalism’, ‘absolute space’), but what we have not really done is say what the difference between absolute and relative space and motion is: just what is at stake? Rynasiewicz (2000) argued that there simply are no constant issues running through the history from antiquity through general relativity theory; that there is no stable meaning for either ‘absolute motion’ or ‘relative motion’ (or ‘substantival space’ vs ‘relational space’). While we agree to a certain extent, we think that nevertheless there are a series of issues that have motivated thinkers again and again. Rynasiewicz is probably right that the issues cannot be expressed in formally precise terms, but that does not mean that there are no looser philosophical affinities that shed useful light on the history and on current theorizing.