Major depressive disorder (MDD), commonly called clinical depression, is the second most common mental health problem, each year affecting millions of people worldwide. It’s a complex condition that has multiple causes, affects the way we think, feel and behavior, and requires a fairly broad treatment approach. It is now known that the negative thinking that characterizes MDD provides an important clue to its causes and treatment. Back in the 1960s and early 1970s, a psychiatrist at the University of Pennsylvania, Dr. Aaron T. Beck, made an important breakthrough in our understanding and treatment of depression called cognitive therapy . Over the years, dozens of studies have found that cognitive therapy is a highly effective treatment for depression. After receiving training in cognitive therapy at Beck’s Center for Cognitive Therapy in Philadelphia, I have been conducting research into the cognitive basis of depression based on Beck’s model of depression. Our studies have explored a number of research questions, mostly focused on the causes of depression and how it differs from other emotional conditions. These include: