Although conventional magnetic resonance imaging (MRI) has greatly increased the understanding of the pathophysiology of multiple sclerosis, its relation to the development of disability is complex. More pathologically specific imaging markers have therefore been sought to try and understand the underlying process that is responsible for the progressive disability that so commonly occurs in multiple sclerosis. Of these the most simple to understand conceptually is the measurement of atrophy, which most probably represents axonal loss. Several recent studies have shown that atrophy is a process closely linked with the progressive phase of multiple sclerosis and worsening disability. Furthermore it has also been shown that atrophy may evolve despite the absence of inflammatory activity as judged by gadolinium enhanced MRI and thus its measurement gives information in addition to that obtained from conventional MRI. Because of new developments in imaging we are now able to measure atrophy reliably and reproducibly. Hence the measurement of atrophy now provides objective markers by which to evaluate putative treatment aimed at preventing disability in multiple sclerosis.