Amyotrophic lateral sclerosis (ALS) is characterized by significant clinical variability. Different subsets are classically individualized: bulbar onset and limb onset ALS, sporadic and familial ALS, ALS-plus syndromes (characterized by the presence of atypical clinical features, e.g. extrapyramidal signs or dementia, in association with the classical phenotype of ALS) and Western Pacific ALS. In addition, ALS-related syndromes include progressive muscular atrophy, primary lateral sclerosis and progressive bulbar palsy. The recognition of ALS subsets and ALS-related syndromes is important in clinical practice since the prognosis may vary depending on the clinical presentation. The prognosis of bulbar-onset ALS is poor compared with the spinal-onset type. Primary lateral sclerosis, defined by pure upper motor neuron findings, has a more benign course than classical ALS. It has also important implications for therapeutic trials to ensure the homogeneity of clinical material since inclusion of atypical forms with different prognoses can skew the outcome analysis. This paper reviews the clinical characteristics of the ALS subsets and ALS-related syndromes.