Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behavior, feelings and sense of well-being. People with depressed mood can feel sad, anxious, empty, hopeless, helpless, worthless, guilty, irritable, ashamed or restless. They may lose interest in activities that were once pleasurable, experience loss of appetite or overeating, have problems concentrating, remembering details or making decisions, and may contemplate, attempt or commit suicide. Insomnia, excessive sleeping, fatigue, aches, pains, digestive problems or reduced energy may also be present.
Depressed mood is a feature of some psychiatric syndromes such as major depressive disorder, but it may also be a normal reaction to life events such as bereavement, a symptom of some bodily ailments or a side effect of some drugs and medical treatments.
Adversity in childhood, such as bereavement, neglect, unequal parental treatment of siblings, physical abuse or sexual abuse, significantly increases the likelihood of experiencing depression over the life course.
Life events and changes that may precipitate depressed mood include childbirth, menopause, financial difficulties, job problems, a medical diagnosis (cancer, HIV, etc.), bullying, loss of a loved one, natural disasters, social isolation, relationship troubles, jealousy, separation, and catastrophic injury.
Adolescents may be especially prone to experiencing depressed mood following social rejection.
Depressed mood can be the result of a number of infectious diseases, nutritional deficiencies, neurological conditions  and physiological problems, including hypoandrogenism (in men), Addison's disease, Lyme disease, multiple sclerosis, chronic pain, stroke, diabetes, cancer, sleep apnea, and disturbed circadian rhythm. It is often one of the early symptoms of hypothyroidism (reduced activity of the thyroid gland).
A number of psychiatric syndromes feature depressed mood as a main symptom. The mood disorders are a group of disorders considered to be primary disturbances of mood. These include major depressive disorder (MDD; commonly called major depression or clinical depression) where a person has at least two weeks of depressed mood or a loss of interest or pleasure in nearly all activities; and dysthymia, a state of chronic depressed mood, the symptoms of which do not meet the severity of a major depressive episode. Another mood disorder, bipolar disorder, features one or more episodes of abnormally elevated mood, cognition and energy levels, but may also involve one or more episodes of depression. When the course of depressive episodes follows a seasonal pattern, the disorder (major depressive disorder, bipolar disorder, etc.) may be described as a seasonal affective disorder.
Outside the mood disorders: borderline personality disorder often features an extremely intense depressive mood; adjustment disorder with depressed mood is a mood disturbance appearing as a psychological response to an identifiable event or stressor, in which the resulting emotional or behavioral symptoms are significant but do not meet the criteria for a major depressive episode;:355 and posttraumatic stress disorder, an anxiety disorder that sometimes follows trauma, is commonly accompanied by depressed mood.
Depressed mood may not require any professional treatment, and may be a normal reaction to certain life events, a symptom of some medical conditions, or a side effect of some drugs or medical treatments. A prolonged depressed mood, especially in combination with other symptoms, may lead to a diagnosis of a psychiatric or medical condition, e.g. of a mood disorder, which may benefit from treatment. Different sub-divisions of depression have different treatment approaches.
The UK National Institute for Health and Care Excellence (NICE) 2004 guidelines indicate that antidepressants should not be used for the initial treatment of mild depression, because the risk-benefit ratio is poor.
- Salmans, Sandra (1997). Depression: Questions You Have – Answers You Need. People's Medical Society. ISBN 978-1-882606-14-6.
- Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). American Psychiatric Association. 2013.
- "NIMH · Depression". nimh.nih.gov. Retrieved 15 October 2012.
- Lindert J, von Ehrenstein OS, Grashow R, Gal G, Braehler E, Weisskopf MG (April 2014). "Sexual and physical abuse in childhood is associated with depression and anxiety over the life course: systematic review and meta-analysis". Int J Public Health 59 (2): 359–72. doi:10.1007/s00038-013-0519-5. PMID 24122075.
- Christine Heim; D. Jeffrey Newport; Tanja Mletzko; Andrew H. Miller; Charles B. Nemeroff (July 2008). "The link between childhood trauma and depression: Insights from HPA axis studies in humans". Psychoneuroendocrinology 33 (6): 693–710. doi:10.1016/j.psyneuen.2008.03.008. PMID 18602762. Retrieved 20 April 2014.
- Pillemer, Karl; Suitor, J. Jill; Pardo, Seth; Henderson Jr, Charles (2010). "Mothers' Differentiation and Depressive Symptoms Among Adult Children". Journal of Marriage and Family 72 (2): 333–345. doi:10.1111/j.1741-3737.2010.00703.x. PMC 2894713. PMID 20607119.
- Schmidt, Peter (2005). "Mood, Depression, and Reproductive Hormones in the Menopausal Transition". The American Journal of Medicine. 118 Suppl 12B (12): 54–8. doi:10.1016/j.amjmed.2005.09.033. PMID 16414327.
- Rashid, T.; Heider, I. (2008). "Life Events and Depression". Annals of Punjab Medical College 2 (1). Retrieved 15 October 2012.
- Davey, C. G.; Yücel, M; Allen, N. B. (2008). "The emergence of depression in adolescence: Development of the prefrontal cortex and the representation of reward". Neuroscience & Biobehavioral Reviews 32 (1): 1–19. doi:10.1016/j.neubiorev.2007.04.016. PMID 17570526.
- Ehret M, Sobieraj DM (February 2014). "Prevention of interferon-alpha-associated depression with antidepressant medications in patients with hepatitis C virus: a systematic review and meta-analysis". Int. J. Clin. Pract. 68 (2): 255–61. doi:10.1111/ijcp.12268. PMID 24372654.
- Murray ED, Buttner N, Price BH. (2012) Depression and Psychosis in Neurological Practice. In: Neurology in Clinical Practice, 6th Edition. Bradley WG, Daroff RB, Fenichel GM, Jankovic J (eds.) Butterworth Heinemann. 12 April 2012. ISBN 978-1437704341
- Saravane, D; Feve, B; Frances, Y; Corruble, E; Lancon, C; Chanson, P; Maison, P; Terra, JL et al. (2009). "Drawing up guidelines for the attendance of physical health of patients with severe mental illness". L'Encephale 35 (4): 330–9. doi:10.1016/j.encep.2008.10.014. PMID 19748369.
- Rustad, JK; Musselman, DL; Nemeroff, CB (2011). "The relationship of depression and diabetes: Pathophysiological and treatment implications". Psychoneuroendocrinology 36 (9): 1276–86. doi:10.1016/j.psyneuen.2011.03.005. PMID 21474250.
- Li, M; Fitzgerald, P; Rodin, G (2012). "Evidence-based treatment of depression in patients with cancer". Journal of clinical oncology : official journal of the American Society of Clinical Oncology 30 (11): 1187–96. doi:10.1200/JCO.2011.39.7372. PMID 22412144.
- Gabbard, Glen O. Treatment of Psychiatric Disorders 2 (3rd ed.). Washington, DC: American Psychiatric Publishing. p. 1296.
- American Psychiatric Association (2000a). Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision: DSM-IV-TR. Washington, DC: American Psychiatric Publishing, Inc. ISBN 0-89042-025-4.
- Vieweg, W. V.; Fernandez, D. A.; Beatty-Brooks, M; Hettema, J. M.; Pandurangi, A. K.; Pandurangi, Anand K. (May 2006). "Posttraumatic Stress Disorder: Clinical Features, Pathophysiology, and Treatment". Am. J. Med. 119 (5): 383–90. doi:10.1016/j.amjmed.2005.09.027. PMID 16651048.
- Zwolinski, Richard and Zwolinski, C.R. Depression and Substance Abuse: The Chicken or the Egg? psychcentral.com
- Kovacs, M. (1992). Children's Depression Inventory. North Tonawanda, NY: Multi-Health Systems, Inc.
- Cheog J et al. for PsychCentral.com. Last reviewed 26 August 2010. Frequently Asked Questions About Depression. Retrieved 11 May 2013
- Staff, UK National Institute for Health and Clinical Excellence (NICE) October 2009. 
- "Depression". National Institute for Health and Care Excellence. December 2004. Archived from the original on 15 November 2008. Retrieved 20 March 2013.