Synopsis For years, computer virus has affected the productive use of computers, often challenging the user and the anti-virus software vendor from preventing or recovering from viral-attacks. Virus attack, hacking, stealing password, browser hijack, spam, and malware are very common major impediments for activities on the Internet. Growing technical sophistication and ingenuity of the masquerader have given birth to another computer malady that has become quite notorious. This is often referred to in the Internet world as ‘Spyware’ in recent times. Spyware is a software program that collects personal information of the users without their formal consent. Unlike viruses and worms, spyware does not usually self-replicate, or intrude into the system directly or spread from one system to the other; instead, it intrudes into a system by deceiving the user or exploiting software vulnerabilities in a system. Once it gets installed in the user system, its implications can range from disturbing to devastating. They are often designed to exploit the infected computers for commercial gain, e.g., selling user and/or system data to online marketing companies, building and trading botnets of compromised computers etc. It propagates using personalization cookies, tracking cookies, Trojans, drive-by downloads, hacking, and piggybacking. Spyware is growing at an alarming pace and needs to be arrested. For this to happen immediate attention is required towards understanding Spyware, its different manifestations, propagation techniques, channels used, and importantly, the countermeasures required at individual, organization, and government levels. This book attempts to do just that by providing insights into this new demon on the Internet.