Malicious web sites that compromise vulnerable computers are an ever-present threat on the web. The purveyors of these sites are highly motivated and quickly adapt to technologies that try to protect users from their sites. This paper studies the resulting arms race between detection and evasion from the point of view of Googleâ€™s Safe Browsing infrastructure, an operational web-malware detection system that serves hundreds of millions of users. We analyze data collected over a four year period and study the most popular practices that challenge four of the most prevalent web-malware detection systems: Virtual Machine client honeypots, Browser Emulator client honeypots, Classification based on domain reputation, and Anti-Virus engines. Our results show that none of these systems are effective in isolation. In addition to describing specific methods that malicious web sites employ to evade detection, we study trends over time to measure the prevalence of evasion at scale. Our results indicate that exploit delivery mechanisms are becoming increasingly complex and evasive.