The diagnosis of ALS is established on the basis of the revised El Escorial criteria revealing involvement of the upper and lower motor neuron. The presence of upper motor neuron signs is recognized by physical examination, but is not always easy to demonstrate. For the patient, early diagnosis reduces the uncertainty and the long waiting period before exclusion test can be performed. Early referral to ALS specialty clinics will have a beneficial effect on the patient's quality of life. Early diagnosis of ALS allows early use of drugs, slowing the rate of disease progression. MR proton spectroscopy and diffusion tensor imaging are potentially useful diagnosis tools but require further analysis of reproducibility in studies with more carefully matched patients and standardized techniques. There is no biochemical marker found in serum or cerebrospinal fluid to establish the definite diagnosis of ALS. Detection of Nogo-A in muscle offers an easy tool for detecting the presence of ALS but further studies are needed to determine the specificity, sensitivity and predictive value of such modifications. When applying standard transcranial magnetic stimulation, examination of several territories improves sensitivity and the probability of detection of infraclinical upper motor neuron dysfunction. The cortical silent period seems most sensitive. The triple stimulation technique (TST) is a very sensitive method but needs to be confirmed by other teams. Some teams have stated that the technique is painful and time consuming for the patient. To date, the corticobulbar tract cannot be studied with TST and we have little experience for the lower limb in very few ALS patients.