This study analyzed the genetic and environmental influences on internalizing and externalizing behavior problems and the nature of their cooccurrence in the national Norwegian twin sample. The sample comprised 526 identical and 389 fraternal same-sexed twin pairs from five to birth cohorts, aged 5-6, 8-9, 12-13, 13-14, and 14-15 years. Behavior problems were assessed by parental ratings on the Child Behavior Checklist. A model of additive genetic, shared, and nonshared environmental influences was fitted to both internalizing and externalizing behavior on four sex and age groups. The considerable covariance, r = .51 to .58, between these traits is accounted for mainly by common environmental components; this effect was most marked in the 5 to 9-year olds. Concordance rates for children scoring above 1 standard deviation from the total sample mean on the internalizing scale only, externalizing problems only than for combined problems.