In the two decades since its first significant appearance, malware has become the most prominent and costly threat to modern IT systems. This chapter examines the nature of malware evolution. It highlights that, as well as the more obvious development of propagation techniques, the nature of payload activities (and the related motivations of the malware creators) is also significantly changing, as is the ability of the malware to defeat defences. Having established the various facets of the threat, the discussion proceeds to consider appropriate strategies for malware detection and prevention, considering the role of modern antivirus software, and its use alongside other network security technologies to give more comprehensive protection. It is concluded that although malware is likely to remain a significant and ever-present threat, the risk and resultant impacts can be substantially mitigated by appropriate use of such safeguards.