Where should be the implementation? Service or BAL?

Hi
I have two solutions Web and Wcf. In WCF I have Service, Entity Framework, BAL and DAL. 
Suppose i am calling a method from WEB. Let it be Add();
Now where should i write the implementation. In Service or BAL?
If i write it in BAL then what should be in Service and vice-versa.
I am confused.
Thanks