Depression is more than simply feeling unhappy or fed up for a few days.
We all go through spells of feeling down, but when you're depressed you feel persistently sad for weeks or months, rather than just a few days.
Some people still think that depression is trivial and not a genuine health condition. They're wrong. Depression is a real illness with real symptoms, and it's not a sign of weakness or something you can "snap out of" by "pulling yourself together".
The good news is that with the right treatment and support, most people can make a full recovery.
How to tell if you have depression
Depression affects people in different ways and can cause a wide variety of symptoms.
They range from lasting feelings of sadness and hopelessness, to losing interest in the things you used to enjoy and feeling very tearful. Many people with depression also have symptoms of anxiety.
There can be physical symptoms too, such as feeling constantly tired, sleeping badly, having no appetite or sex drive, and complaining of various aches and pains.
The severity of the symptoms can vary. At its mildest, you may simply feel persistently low in spirit (read about low mood), while at its most severe depression can make you feel suicidal and that life is no longer worth living.
For a more detailed list, read more about the symptoms of depression.
Most people experience feelings of stress, sadness or anxiety during difficult times. A low mood may improve after a short time, rather than being a sign of depression. Read more information about low mood and depression.
If you've been feeling low for more than a few days, take this short test to find out if you're depressed.
When to see a doctor
Sometimes there is a trigger for depression. Life-changing events, such as bereavement, losing your job or even having a baby, can bring it on.
People with a family history of depression are also more likely to experience it themselves.
But you can also become depressed for no obvious reason.
Find out more about the causes of depression.
Depression is quite common and affects about one in 10 of us at some point. It affects men and women, young and old.
Depression can also strike children. Studies have shown that about 4% of children aged five to 16 in the UK are anxious or depressed.
Treatment for depression involves either medication or talking treatments, or usually a combination of the two. The kind of treatment that your doctor recommends will be based on the type of depression you have.
Read more about the treatment of depression.
Living with depression
Many people with depression benefit by making lifestyle changes such as getting more exercise, cutting down on alcohol, stopping smoking and eating more healthily.
Self-help measures such as reading a self-help book or joining a support group are also worthwhile.
Find out more about how self-help and improving your lifestyle can help you beat depression.
Read how stopping smoking can improve your mood.
You can read other people's experience of depression in our comments section below.