Patients with mood disorders experience substantial challenges in their lives, often over long-term periods and despite receiving treatment. Provision of clinical care for mood disorders involves direct monetary costs. Illness also leads to indirect socioeconomic costs due to reduced work capacity. The absence of these patients from wider economic activity within society is another indirect cost. Estimating the impact of mood disorders in monetary terms mainly relies on administrative records, patient surveys, and mathematical models. Although estimations may vary between studies depending on methodology, annual economic cost of major depressive disorder and bipolar disorder in the UK may exceed £8 billion and £7 billion, respectively, with the majority of this cost accounted for by lost production rather than provided healthcare. Other indirect costs are commonly ignored and require further research. Cost of illness studies may serve as the basis for economic evaluations (e.g., cost-effectiveness analyses) of interventions targeting mood disorders.