This paper provides the first comprehensive analysis of the effects of "Zero Tolerance" (ZT) Drunk Driving Laws--which set very low legal blood alcohol limits for individuals under age 21--on self-reported alcohol use and drunk driving using data from the 1984 to 2001 Behavioral Risk Factor Surveillance System (BRFSS). I estimate two-way fixed effects models of alcohol-related behaviors of 18-20-year-olds that can condition on unobserved differences across states that may be correlated with determinants of drinking and drunk driving, and I use 22-24-year-olds as a control group. Results indicate that the laws reduced heavy episodic drinking (five or more drinks at one sitting) among underage males by 13%. This result is supported by models that use variation in treatment intensity induced by differences in body weight. I find mixed evidence of ZT effects for females, and no robust effects on drinking participation or drunk driving for either sex.