Editorial: What is depression?

'Depression is a common, debilitating, and potentially lethal disorder.' This is a standard opening to many a scientific paper on depression. And it is often followed by some very grim statistics. Over 300 million people in the world are estimated to live with depression, and the disorder is ranked by WHO as the single largest contributor to global disability. Most worryingly, adolescents with major depressive disorder are up to 30 times more likely to commit suicide. Yet, what exactly is depression? Do all 300 million depressed people in the world suffer from the same thing, with the same aetiology? Is depression one disorder that comes in different shades of severity or is it best thought of as heterogeneous mix of problems that we have given one common name to? And, more practically, should we be treating all depression in similar ways?