DOT-regulated employers must maintain a random drug testing program selecting at least 50% of the covered employee pool for drug testing and 10% for alcohol testing per calendar year. Selection must use a scientifically valid method (typically a computer-based random number generator). Each selection cycle must be unannounced, and selected employees must proceed to the collection site immediately upon notification. Non-DOT random testing is governed by state laws, which may have different requirements for selection rates, notice, and implementation.