The first which comes to the minds of many is: Secure the border. I think this one competes with that for most important: At least make it seem like a strong possibility that we will actually enforce our immigration laws. If illegal immigrants seriously believe that they could be deported simply for being here illegally and if they seriously believe that we'll start enforcing laws in connection with the hiring of undocumented workers, many will choose to go home so that they can do it on their own terms. And a lot more potential illegal immigrants will decide not to risk coming here at all if they think there is a significant chance they could be sent back or not find a job. Maybe then we might only have say 6 or 8 million to worry about the costs of, instead of 11 million or whatever the current number is.