We present a study of Fake Anti-Virus attacks on the web. Fake AV software masquerades as a legitimate security product with the goal of deceiving victims into paying registration fees to seemingly remove malware from their computers. Our analysis of 240 million web pages collected by Google's malware detection infrastructure over a 13 month period discovered over 11,000 domains involved in Fake AV distribution. We show that the Fake AV threat is rising in prevalence, both absolutely, and relative to other forms of web-based malware. Fake AV currently accounts for 15% of all malware we detect on the web. Our investigation reveals several characteristics that distinguish Fake AVs from other forms of web-based malware and shows how these characteristics have changed over time. For instance, Fake AV attacks occur frequently via web sites likely to reach more users including spam web sites and on-line Ads. These attacks account for 60% of the malware discovered on domains that include trending keywords. As of this writing, Fake AV is responsible for 50% of all malware delivered via Ads, which represents a five-fold increase from just a year ago.