Depression is a very frequent illness with a lifetime prevalence of 33.6% in the population of the Province of Liege. In addition to the marked personal and family suffering which is associated, depression is responsible for important socio-economic costs evaluated to more than 40 billions Belgian francs per year in Belgium, what makes it the most expensive illness after cardio-vascular disorders. Depressive illness is associated to particularly high indirect costs, depending on sick leaves, loss of productivity and suicide. These indirect costs are about 6 times more important than direct costs resulting from the treatment of the illness. Depressive illness remains unsatisfactorily treated and the use of available therapeutical methods should be optimized.