Hey can anybody help with polar coordinates?

when you are doing a double integral and you have x^2+y^2=4. This means that r is equal to 2 so one of the integrals is bounded by the limits 2 and 0. however i am not sure how to find out the other limits to the integral. Apparantly it is 2(pi) (not sure how to get the symbol up!) and 0. but i dont know how to get this. Can anybody help?