scp weirdness

I'm having a problem using scp in a shell script. It works fine when I run it from the command line. However, when I use it in a script it prompts for a password. If I use ssh in a script it works as well. What is different about scp, why does it behave differently in a script then on the command line and how can I remote copy a file in a script without needing a password? TIA.