Departments may wish to hire or retain faculty and highly specialized scientific staff who are non-immigrants who are eligible for TN (Trade National) visa status.
The TN Visa
"The North American Free Trade Agreement (NAFTA) created special economic and trade relationships for the United States, Canada, and Mexico. The TN nonimmigrant classification permits qualified Canadian and Mexican citizens to seek temporary entry into the United States to engage in business activities at a professional level." (USCIS)
Permanent residents of Canada or Mexico are not eligible for the TN visa. Only citizens of Canada or Mexico qualify.
TN status may be granted for an initial period of up to three years and may be extended indefinitely in three-year increments.