During 20 last years, there has been an exponential growth in the literature related to quality of life (QoL). The usual indicators of results are not always responsive to new strategies or new systems of care organization. While assessment of QoL by patients and by the medical community is an old concern, the issue of standardized instruments is more recent. Despite the need, the use of QoL questionnaires remains rare because of the lack of sufficiently dedicated tools. In the specific field of motoneuron diseases, a search of the literature should be helpful to clarify the definition of quality of life. It should also be helpful in determining the limits of the debate on quantification of an impression considered to be subjective. The goal is to clarify methods for standardizing QoL evaluation, to specify nosographic notions affecting the choice of measuring instruments, to recall the prerequisite of validation before use, and finally to specify the fields of application of such tools. Thus in the field of motoneuron diseases, recognizing the limitations of QoL instruments should enable a more enlightened approach to overall assessment of the burden of these diseases.