Amyotrophic lateral sclerosis (ALS) is a progressive degeneration of upper and lower motor neurons. In the absence of any validated biological marker, the diagnosis of ALS depends upon recognition of characteristic symptoms and signs together with supportive electrophysiological findings. The diagnosis of ALS is easy to recognize in its fully developed form but during the early stages both false positive and false negative diagnoses are common. In clinical practice, diagnostic difficulties mostly arise with patients who present either with only upper motor neuron, or with only lower motor neuron signs. It may be difficult to distinguish ALS with clinically predominant lower motor neuron involvement from alternative diagnoses including spinal atrophies of adult onset, Kennedy's disease, inclusion body myositis and motor neuropathies with conduction blocks. The diagnosis of ALS related syndromes (progressive muscular atrophy, primary lateral sclerosis and progressive bulbar palsy) requires the elimination of alternate diagnoses. This paper reviews the main characteristics of diseases mimicking ALS and the atypical subsets of ALS.