Cap and Trade Solutions

I had to look up what "cap and trade" meant and it is exactly as it sounds.
A company has so many pollution credits they get to use if they are heavy polluters or trade if they pollute less. The goal being to keep pollution numbers low, thus not damaging the environment.
I probably would not have come up with that idea myself but whatever works. Notice that saving the environment is different from not damaging the environment.
Could someone tell me what the current argument or problem is with this system of pollution control?
It is a hot button issue.