With statistics suggesting that around one in four adults will experience a mental illness at some point each year in the UK, the chances are, you might know someone who's suffering right now.
Depression can be a particularly difficult condition to understand, and you may be confused about how best to help a close friend when they're going though this form of mental illness.
We've spoken to an array of experts, as well as some people who've been personally affected, to deliver you the best advice on how to support your best friend when they're depressed.
It goes without saying, but make sure your friend is receiving medical help as well as your love and support.
It's key that they get professional advice and attend regular GP appointments.
Caroline Hounsell, director of partnerships product development and training at Mental Health First Aid England said: 'Let them know that you're there to talk and that there are sources of further support and resources available.
It can be easy to make snap judgements about why someone is depressed or to assume you know how to fix their problems, but try to be impartial.
Caroline said: 'try and listen non-judgementally and empathetically and give them your full focus.
It's also worth bearing in mind that the symptoms of depression manifest themselves in unusual ways.
Counsellor Katerina Georgiou said: 'Some people will be noticeably withdrawn and show signs of weight gain or weight loss, lack of sleep, lack of energy, but not everyone will fit this picture.
Contacting your friend might feel awkward at first, but try to persevere, especially if they're purposely isolating themselves.
Jon Imondi went through a period of depression and said regular contact was essential for his recovery.
It can be tempting to intervene when you see your friend struggling with everyday tasks but Katerina says finding the balance between being helpful and overbearing is key.
She said: 'To give an example, if they haven't done their dishes, don't do the dishes for them, but equally don't pressurise them to do it or get angry.
Instead of dragging them to a pub, restaurant or other crowded place, suggest you both go for a walk.
'The last thing many would want to do is to be surrounded by people, it's way too much, especially if anxiety is kicking in,' according to depression sufferer Jon.
Blogger Shawntel says some of the best support is just being in the same room as your best friend without having to say a word.
The truth is that most of us with depression just want someone to listen, not someone who claims to have all the answers.
'If you have a friend suffering from depression, the main thing is to understand first of all is that you're not responsible for it and you can't make it go away'.
Although this can feel frustrating, don't feel disheartened.
So instead of thinking about what to do to try and fix the situation, counsellor Katerina recommends instead thinking about how to 'be'.
Visit Mind for more advice on caring for someone with depression.