The Web is increasingly being accessed by portable, multi-touch wireless devices. Despite the popularity of platform-specific (native) mobile apps, a recent study of smartphone usage shows that more people (81%) browse the Web than use native apps (68%) on their phone. Moreover, many popular native apps such as BBC depend on browser-like components (e.g., Webview) for their functionality. The popularity and prevalence of web browsers on modern mobile phones warrants characterizing existing and emerging threats to mobile web browsing, and building solutions for the same. Although a range of studies have focused on the security of native apps on mobile devices, efforts in characterizing the security of web transactions originating at mobile browsers are limited. This dissertation presents three main contributions: First, we show that porting browsers to mobile platforms leads to new vulnerabilities previously not observed in desktop browsers. The solutions to these vulnerabilities require careful balancing between usability and security and might not always be equivalent to those in desktop browsers. Second, we empirically demonstrate that the combination of reduced screen space and an independent selection of security indicators not only make it difficult for experts to determine the security standing of mobile browsers, but actually make mobile browsing more dangerous for average users as they provide a false sense of security. Finally, we experimentally demonstrate the need for mobile specific techniques to detect malicious webpages. We then design and implement kAYO, the first mobile specific static tool to detect malicious webpages in real-time.