Is it illegal to house an illegal alien?

Technically, there is a federal law which prohibits anyone from aiding or abetting a known illegal alien. I believe the law dates back to the 1950s. However, there is no instance in which that law was ever enforced with respect to simply allowing a friend or family member to live with you. Consequently, it would be difficult for the federal government to begin enforcing that law, since defendants would argue selective enforcement.

However, some recent state laws (notably the Alabama law taking effect September 1, 2011) make it a misdemeanor under state law to provide shelter to anyone when you know his or her presence in the United States is illegal.