The proposed study is a two-phased clinical trial of lithium for the treatment of aggression in conduct disorder. Both phases are double-blind and placebo-controlled with randomization and employ a parallel groups design. Phase 1 contains a short-term 8-week controlled trial, with twice as many subjects randomized to lithium as placebo, increasing the pool of potential lithium responders to continue to Phase 2. In Phase 2, lithium responders from Phase 1 enter a 6-month long-term controlled trial. Every attempt is made to define responders to lithium.