Cognitive impairment (CI) is a serious complication of multiple sclerosis (MS), and the domains affected are well established, but new affected domains such as theory of mind are still being identified. The evidence that disease-modifying therapies (DMTs) improve and prevent the development of CI in MS is not solid. Recent studies on the prevalence of CI in MS among people treated with DMT, although not as solid as studies completed prior to DMT introduction, suggest that CI remains a problem even among people on DMTs and that CI occurs frequently even at the very earliest stages of MS. Functional MRI studies and studies using diffusion tractography show that the impact of lesions on cognition depends on the particular cortical networks affected and their plasticity. Cognitive rehabilitation and L-amphetamine appear promising symptomatic treatments for CI in MS, while, cholinesterase inhibitors and memantine have failed, and data on Ginkgo and exercise are limited. We need more work to understand better CI in MS and develop treatments for this serious complication of MS.