I study economics.
Monopolies are often forbidden, cause they are not efficent. Since they can set their own prices, they usually set higher prices(and sell less products). This is bad for consumers, who consumes less (that's always bad) but have to pay more. But I guess that kind of goes without saying, and is besides your assignment.
But it is important to remember that monopolies aren't always bad. Natural monpolies are the only way to go in some industries (railroads, phone networks, industries with very high initial costs), as it would be a great waste of resources having several equal competitors there. Other times companies needs to be allowed to be a monopoly, as an incentive to research and develop costly but important things. These monopolies will be protected by laws (as patents).
In Norway we have a department called "konkurransetilsynet" (goverment run) who looks our for cartels and "illegal" monopolies. I would guess that most countries with functional economies have the same. Opec, a cartel, should maybe be illegal, but there is no organ controlling countries in the same way.