Root Login Problem

When I type su in the terminal and type in my correct password, it gives me an Authentication Failure. Why?

how are you connected to the machine, are you using local console, ssh, telnet etc...

Just in case you didn't know, the "su -" password is the root password and not a regular user password. Always use su - the hyphen (-) is important as it makes sure you get all of root permissions and file paths.

or the sudo command might work better...