Short Message Service (SMS) is widely used to provide communication between users around the world. In the last years, it has become a lucrative way for cybercriminals to infect mobile devices with botnets and launch malicious attacks without the user's knowledge such as: SMS spam and SMS premium rate fraud. SMS spam is one of the dangerous malicious activities as the user is charged for each sent SMS spam. Existing solutions to this issue are no longer adequate as they are either costly in terms of resources, inefficient, or require user intervention with some knowledge on security. In this book, we design, analyze, and implement a system that detects any attempt to send out malicious SMS spams from the mobile device. The proposed system combines the rule-based and the content-based approaches into one detection process. It achieves good performance in terms of detection time and execution cost, and offers a good tradeoff between resilience against attacks and user interaction with the system. This book is intended for multiple audiences: academic, professional, students, and anyone else who is interested in IT security, and particularly in mobile security and botnet detection.