Botnets are emerging threat with hundreds of millions of computers infected. A study shows that about 40% of all computers connected to the internet in the world are infected bots and controlled by attackers. This article isa survey of recent advances in botnet research. The survey classifies the botnet research into three areas: understanding botnets, detecting and tracking botnets, and defending against botnets. While botnets are widespread, the research and solutions for botnets are still in their infancy. The paper also summarizes the existing research and proposes future directions for botnet research.