WE live in close quarters with them and more often than not are thrust together for financial reasons, convenience, friendship or desperation.
They help cover the rent and share household duties such as cooking dinner or cleaning up - in theory.
In practice, they may come up with all sorts of excuses to dodge rent day, never prepare a meal but take the lion's share of it, and live like a slob.
You know the ones - those who parade around the house naked on "No Clothes Wednesday", flirt with your boyfriend and eat all your food.
The ones who steal your clothes, stain the carpet and never take out the rubbish.
The ones who refuse to move out even when, in total frustration, you find someone else to move into their room.
Bad memories of flatmates may linger longer than those of failed relationships.
In short, they can be more trouble than they're worth.
Ask the question: "Have you ever had a flatmate from hell?" and be prepared for a volley of responses - sometimes detailing the most ridiculous behaviour displayed by grown men and women.
These are the ones known as "The Crazies".
They are the flatmates who really step outside the boundaries of decency.
They are the ones who promise to get a job but never do and instead spend every waking hour on your couch, watching your plasma TV, flicking through your magazines and inviting their other unemployed friends over for lunch.
Or maybe they are the ones who are just too perfect for their own good.
One victim said she lasted two-and-a-half years before she pulled the pin on her flatmate from hell.
"She was a neat freak and would lose it if the magazines where not in the correct stack shape (biggest to smallest on top)," the woman said.
"If I left my laptop on the side table in the lounge, she would make a point of saying: 'Are you really going to leave that there?'"
The tension did not end there, either.
The hellish minion also was a member of the Fashion Police.
"She would go through my wardrobe and put on my bed what I should be wearing," the former flatmate revealed.
"Then, I had blonde-ish hair but she said it didn't suit me so I gave in to her dyeing it for me … the same shade as hers."
Cross the Pacific Ocean and the story seems to be a similar one, as Sarah Mahoney discovered while in the United States.
"I actually had a slob for a roommate when I was studying in America," she said, adding that the extent of the mess created even encroached on her bedroom.
"I ended up staying with another friend at his apartment until she cleaned up our room because of the amount of mess she left everywhere," Sarah said.
But there are ways to avoid these hellish living conditions.
REIQ Sunshine Coast spokesman Lloyd Edwards offers some useful advice about sharing accommodation.
"The best thing home sharers can do is to set ground rules in writing from day one and signed by all parties," he said.
He recommended keeping a record of who paid the bond, how the rent was allocated and setting rules for removing someone from the house.
The Rental Tenancies Authority also has some helpful advice if you are looking for the ideal flatmate:
Interview anyone you are thinking of sharing your home with, regardless of how well you know them.
Even your best friend can turn out to be "mysterious, kooky, and altogether spooky".
Lay down the law from day one.
If you want your home to be kept clean, let your flatmate know what the expectations are before letting them move in.
If you are already renting and want to move someone in as a flatmate, make sure the arrangement has been authorised by the landlord or property manager.
This will officially make you the head tenant and give you the same rights as a landlord.
Last but not least, trust your gut instinct. If something "smells off" about the potential flatmate, you are probably right and are better off on your own.