See Psalm 42 concerning depression.
Depression is a mental illness  in which people feel sad for no reason at all or simply find it difficult or impossible to deal with the normal things of life. Pessimism is very often the cause of depression. 
It affects approximately 10% of people. The feeling of depression is something that many people experience from time to time  and can even help individuals to recognize and deal with problems in their lives. In clinical depression or Bipolar disorder, the feeling of depression is worse, goes on for longer and makes it difficult or impossible to deal with the normal things of life. Although depression can be triggered by life events such as childbirth (see Postpartum depression), sufferers also often describe feeling sad for no reason at all.
Clinical depression can manifest itself in many different ways.
- Feelings of unhappiness that don't go away
- Wanting to burst into tears for no reason
- Loss of interest in things
- Being unable to enjoy things
- Feeling restless and agitated
- Loss of self-confidence
- Feeling useless, inadequate and hopeless
- Feelings of guilt
- Feeling more irritable than usual
- Thoughts of suicide.
- Inability to think positively or hopefully
- Great difficulty in making even simple decisions
- Poor or lack of concentration
- Significant, unintentional loss of appetite and weight
- Difficulty in getting to sleep
- Waking earlier than usual
- Sleeping too much
- Feeling exhausted
- Loss of libido
- Difficulty in starting or completing things, even everyday chores
- Crying a lot or feeling the need to cry, but not being able to
- Social isolation/withdrawal
The two best-known theories are, first, that depression is caused by a chemical imbalance in the brain; second, that gloomy thinking causes depression. According to the second theory, it is the depression that causes the chemical imbalance (not the other way around).
- The cognitive theories of depression are among the most clearly articulated and well-supported models of depression. According to these theories (e.g., Abramson, Metalsky, & Alloy, 1989; Nolen-Hoeksema, 1991), some individuals have a cognitive vulnerability that interacts with stress to produce depression. Specifically, people are vulnerable to depression because they have a tendency to generate interpretations of stressful life events (and dysphoric moods) that have negative implications for their future and for their self-worth. 
- National Institute of Mental Health
- Royal College of Psychiatrists
- WebMD Depression Health Center
- National Suicide Prevention Lifeline
- "Depression is a mental illness that can affect mood, cognition, and physiological functioning. Each year about 21 million American adults (nearly 10% of the population) suffer from depression or a related illness. Robins, L.N. & Regier, D.A. (1990). Psychiatric Disorders in America: The Epidemiologic Catchment Area Study. New York: The Free Press.
- American Psychiatric Association (1994). Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV). Washington, DC: American Psychiatric Association.
- SAVE - The Link Between Depression and Suicide