When you are about to enter a roundabout...
Vehicles inside the roundabout always have priority; entering without yielding can cause collisions.
If you are driving inside a roundabout...
Roundabouts operate on a give way to the left(or right, depending on the country) rule to maintain continuous flow.
When approaching a roundabout, and traffic is heavy...
Rushing into a roundabout without a safe gap violates priority and can lead to crashes.
If another driver enters the roundabout without yielding...
Safety comes first, even when you have the right of way.
English 
Spanish 