Since 09 you are supposed to have a passport but I & many people I know have misplaced my passport and had to cross, you absolutely must have a U.S. Issued I.d. And proof of citizenship , if you don't have those they detain you until they figure it out, which is something no one wants to go through but indeed is necessary. Some of the border patrol agents have told me that if you have the 2 documents I did and nothing to hide (warrants, etc.) they are going to let you through but if everyone did what they were supposed to it would be much faster & easier on all. (Btw I didn't want to cross not having my passport but had a surgery check up that day so it was something I couldn't miss) long story why I couldn't find it but I have kids if that tells you something.