What ID to bring to Europe

Hi, my friend and I will be leaving for about a month to Europe and we're wondering if it's necessary to bring all ID to Europe. I was planning on my passport, driver's license and international license, but all she has is her passport. She doesn't have a license. Should she bring her birth certificate? Should I?

Thanks!
Meg

when i travelled to italy last year for 4 weeks the only identification i had on my was my passport. your drivers licence is really null and void unless you pay for an international license while you are in europe.

Thanks for the reply!