Cognitive impairment is a common and disabling feature of Multiple Sclerosis (MS), including early MS, and may even pre-date any physical symptoms. It contributes even more to withdrawal from work than physical disability. Here, we provide an overview of cognitive impairment in MS, particularly in early MS where it is most commonly under-reported and under-treated. We address the presenting features of CI, its impact on quality of life, and its validated assessments (in particular the use of Brief International Cognitive Assessment in MS for use in a clinical setting). We review the insights radiology has given us into the pathogenesis of cognitive impairment in MS, particularly in early CI and in cognitively preserved MS patients. We review current treatments for cognitive impairment, primarily cognitive rehabilitation. We address the evidence for its associated co-morbidities, which may exacerbate or trigger CI, and should therefore be addressed early in the disease course (smoking, alcohol, mood, fatigue and potential co-existing sleep disorders, exercise, and vitamin D). The article supports the importance for early recognition and management of cognitive impairment in MS, before it becomes an established and irreversible entity.