Depression is marked by a profound and constant sense of hopelessness and despair – symptoms that interfere with a person's ability to work, study, sleep, eat, and enjoy once pleasurable activities. It is a complex disorder that can be attributed to many different factors, including emotional, environmental and biological influences. It has been shown to run in families and it tends to recur.

The causes of depression are not understood. Early biological hypotheses considered that depression was caused by an imbalance of important chemical messengers in the brain called neurotransmitters. Current antidepressant therapy alleviates many of the symptoms of depression, perhaps in part by correcting some of the prior chemical imbalances. However, just because antidepressants help some depressed patients does not mean that a chemical imbalance was the cause of depression, nor does it help those who do not respond to current antidepressants. And, even if the imbalance view were partially correct, we do not understand why it takes 1-2 weeks after treatment starts before notable recovery is detected.

There is a growing body of evidence which suggests that depression may actually be a special kind of neurodegenerative disorder. We know that some people suffering from depression have structural changes in their brains compared to people who do not suffer from depression. Imaging studies and postmortem examination of the brains of depressed patients have consistently found a loss of neurons in specific brain regions, such as the hippocampus – a small part of the brain that is vital to the storage of memories. In fact, the hippocampus is smaller in women with a history of depression than in those who've never experienced this illness. Currently, attention is being paid to the concept that antidepressant medications may help bolster an inadequate formation of new hippocampal neurons, a concept that was totally unexpected as few as 5 years ago.