Using a nationally-representative panel data set of U.S. high school students (AddHealth data) that contains a relatively large sample of siblings and twins, the paper investigates the impacts of gun availability at home and individual drug use on robbery, burglary, theft and damaging property for juveniles. Using a variety of fixed-effects models that exploit variations over time and between siblings and twins, the results show that gun availability at home increases the propensity to commit crime by about two percentage points for juveniles but has no impact on damaging property. The results indicate that it is unlikely that gun availability is merely a measure of the unobserved home environment because gun availability does not influence other risky or bad behaviors of juveniles such as smoking, drinking and fighting, being expelled from school, lying, and having sex. No support is found for the hypothesis that gun availability decreases the propensity for being victimized. In fact, the results show that having access to guns increases the probability of being cut or stabbed by someone and of someone pulling a knife or gun on the juvenile. Estimates obtained from models that exploit variations over time and between siblings and twins indicate that drug use has a significant impact on the propensity to commit crime. We find that the median impact of cocaine use on the propensity to commit various types of crimes is 11 percentage points. The impact of using inhalants or other drugs is an increase in the propensity to commit crime by 7 and 6 percentage points, respectively.