North Korea has conducted five nuclear tests and is believed to be rapidly increasing the size and sophistication of its nuclear arsenal. Increased sophistication, particularly the ability to miniaturize nuclear devices, requires more nuclear tests. The size of the arsenal is limited primarily by the stockpile of fissile material ― plutonium and highly enriched uranium (HEU). Current plutonium inventories are estimated with moderate confidence to be in the range of 20 to 40 kg, sufficient for the manufacture of 4 to 8 plutonium bombs. HEU inventories are estimated with much greater uncertainty to be in the range of 200 to 450 kg, sufficient for 10 to 25 HEU bombs. Annual production rates are estimated to be less than 6 kg of plutonium and ~150 kg HEU.