Amyotrophic lateral sclerosis is increasingly recognized to be a complex multisystems disorder both at the level of its pathobiology and in the breadth of non-motor manifestations that can accompany it. Paramount among these are disorders of frontotemporal function which can be associated with syndromes of behavioural, cognitive or executive dysfunction or manifest as a frontotemporal dementia (FTD). While these may occur in isolation and precede the development of motor deficits, more commonly they insidiously onset following the initial neuromuscular dysfunction. The earliest clinical manifestation is a loss of verbal fluency, disproportionate to impairments in oromotor control. There is good correlation between the presence of a syndrome of frontotemporal dysfunction and alterations in brain structure or function as identified with a wide variety of neuroimaging techniques and which reflect a frontotemporal lobar degeneration (FTLD). Although the cause(s) of this process remain to be defined, as with the clinical heterogeneity, there is likely to be significant biochemical heterogeneity. This includes alterations in tau protein metabolism which are present in a proportion of familial and sporadic ALS cases, as well as the western Pacific variant, and recently described alterations in the metabolism of the TAR DNA binding protein 43 (TDP-43).