This is a retrospective study of approximately one million subjects randomly selected from an anonymized database National Health Insurance Research Database (NHIRD). Participants aged 18 years or older who have had neither a depression diagnosis nor a dispensing of an antidepressant medication in the last four months of 2004 will enter the study when they receive in 2005 a depression diagnosis and a dispensing of an antidepressant medication within 30 days of each other and will be followed up to 8 years or until they have 4 months with neither a depression diagnosis nor a dispensing of an antidepressant medication. Treatment Resistant Depression (TRD) Incidence, TRD prevalence by age and sex and duration of clinical episodes will be estimated.