Formatting external harddrive

Hey guys,
Not sure if this is the right area or not. But, I have yet to figure out how i can format an external harddrive to be able to read/write in both Windows and Mac. Can anyone help me with this?
Thanks!

Hi,

I have an Iomega external USB drive that came formatted as FAT for Windows and I use this on both my Mac and my Windows machine.

Both Windows machines and Macs can read and write to FAT format disks. However, getting Windows PCs to read and write to Mac HFS disks requires some additional software.