The role of environmental influences in the aetiopathogenesis of amyotrophic lateral sclerosis (ALS) has exercised minds since the 19th century. Despite strong hunches that environmental agents might be implicated in the causation of ALS, research seeking more objective evidence has generally yielded results which have been confusing and difficult to understand in terms of a unitary aetiological hypothesis. This review attempts to draw this evidence together in the context of a semi-systematic review of the literature. Potential physical influences are described as well as the better known chemical factors. The interface with recent advances in molecular genetics is reviewed as well as foci of ALS variants as they occur in localized areas in various parts of the world. There is a discussion of large and small area geographical clustering and the review concludes by presenting a general view of the possible significance of these--at times--confusing pointers in the context of other current theories of the aetiology of ALS.