The growing popularity of Android applications has generated increased concerns over the danger of piracy and the spread of malware, and particularly of adware: malware that seeks to present unwanted advertisements to the user. A popular way to distribute malware in the mobile world is through repackaging of legitimate apps. This process consists of downloading, unpacking, manipulating, recompiling an application, and publishing it again in an app store. In this paper, we conduct an empirical study of over 15,000 apps to gain insights into the factors that drive the spread of repackaged apps. We also examine the motivations of developers who publish repackaged apps and those of users who download them, as well as the factors that determine which apps are chosen for repackaging, and the ways in which the apps are modified during the repackaging process. Having observed that adware is particularly prevalent in repackaged apps, we focus on this type of malware and examine how the app is modified when it is injected in an app’s code. Our findings shed much needed light on this class of malware that can be useful to security experts, and allow us to make recommendations that could lead to the creation of more effective malware detection tools, Furthermore, on the basis of our results, we propose a novel app indexing scheme that minimizes the number of comparisons needed to detect repackaged apps.