Increased awareness of cognitive and behavioural change in amyotrophic lateral sclerosis has been driven by various clinic-based and population-based studies. A frontotemporal syndrome occurs in a substantial proportion of patients, a subgroup of whom present with frontotemporal dementia. Deficits are characterised by executive and working-memory impairments, extending to changes in language and social cognition. Behaviour and social cognition abnormalities are closely similar to those reported in behavioural variant frontotemporal dementia, implying a clinical spectrum linking amyotrophic lateral sclerosis and frontotemporal dementia. Cognitive impairment should be considered in clinical management, but few specialist assessment resources are available, and thus the cognitive status of most patients is unknown. Standard assessment procedures are not appropriate to detect dysfunction due to progressive physical disability; techniques that better measure the problems encountered by this group of patients are needed to further establish disease effects. Screening instruments are needed that are validated specifically for amyotrophic lateral sclerosis, encompass the heterogeneity of impairment, and accommodate physical disability.