Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder involving primarily motor neurons in the cerebral cortex, brainstem and spinal cord. In the absence of any biological marker, the diagnosis of ALS is based on clinical analysis, combined with the results of electromyography. Consensus diagnosis criteria (El Escorial criteria) have been developed to define workable and internationally acceptable guidelines for the diagnosis of ALS. A combination of lower and upper motor neuron signs with evidence of spread within a region or to other regions is required. The diagnosis of ALS has been categorized into various levels of certainty depending on the presence of upper motor neuron and lower motor neuron signs together in the same topographical anatomic region (brainstem and cervical, thoracic or lumbosacral spinal cord). Clinical types and patterns of ALS have been defined. The emerging concept of "ALS plus" is characterized by the presence of atypical clinical features, e.g. extrapyramidal signs or dementia, in association with the classical phenotype of ALS. This paper reviews the classical and atypical clinical features of ALS.