This study will investigate the efficacy of brivaracetam monotherapy in patients 2 to 25 years of age inclusive, diagnosed with childhood or juvenile absence epilepsy. The study will last about 5 months. Subjects may take a placebo during two 2-week periods, and will take brivaracetam during the rest of the 5 months. There will be 13 clinic visits. After the 5 months, subjects may be eligible to participate in a long-term folllow-up study and continue taking brivaracetam.