Multiple sclerosis (MS) is typically considered to be a chronic inflammatory-demyelinating disease of CNS white matter. In the past decade, however, pathological and MRI studies have shown that lesions are often located in the gray matter, especially in the cerebral cortex. The histopathological characteristics of these cortical lesions differ substantially from lesions located in the white matter, which suggests location-dependent expression of the MS immunopathological process. Double inversion recovery imaging--an MRI technique that selectively images gray matter and lesions--has enabled researchers to image cortical lesions in vivo. Double inversion recovery studies have shown that cortical lesions can be detected at the earliest clinical stages of MS, and cortical lesion burden positively correlates with the severity of physical and cognitive impairments. These gray matter lesions are also independent predictors of subsequent disease evolution. This Review provides a summary of the main histopathological and MRI findings with regard to cortical lesions in MS, and indicates that increasing our understanding of cortical lesions has increased our knowledge of MS pathobiology.