The concept of quality of life appears to be easy to understand but, actually, is very complex when it comes to define it. Several notions are synonymous and often used indiscriminately such as happiness, life satisfaction and well-being. However, quality of life is dependant on several factors that, according to their presence or their absence, can change its perceptions and evaluation. Besides the difficulties of definition, methodological difficulties are added for its evaluation: how to measure a multi-factor concept whose definition is complex and non-consensual? What about its assessment in the elderly? Are the difficulties of definition and evaluation the same in studies involving elderly dementia patients? Starting from the historical context of quality of life we particularly discuss its definition and assessment in elderly patients with dementia. (PsycINFO Database Record (c) 2016 APA, all rights reserved)