Botnets, which are networks formed by malware-compromised machines, have become a serious threat to the Internet. Such networks have been created to conduct large-scale illegal activities, even jeopardizing the operation of private and public services in several countries around the world. Although research on the topic of botnets is relatively new, it has been the subject of increasing interest in recent years and has spawned a growing number of publications. However, existing studies remain somewhat limited in scope and do not generally include recent research and developments. This paper presents a comprehensive review that broadly discusses the botnet problem, briefly summarizes the previously published studies and supplements these with a wide ranging discussion of recent works and solution proposals spanning the entire botnet research field. This paper also presents and discusses a list of the prominent and persistent research problems that remain open.