Including two header files in c++

I have a project I'm working on and I'm currently working on the header files. For simplicity let's say I have two header files so far. fileX.h and fileY.h

The code I have in both is just class definitions. But, fileX needs fileY's class definitions and fileY needs fileX. I tried having inside of fileX a #include "fileY.h" and have a include "fileX.h" inside of my fileY, but I'm getting an error that gets in a never ending loop. How can I fix this ? Thanks