This book summarizes several researches and reports about spam. Many users underestimate the losses caused by spam and assume the severity of the problem by the amount of spam they receive in their inbox. However, the total worldwide financial losses caused by spam are estimated at several billion USD per year. The problems of spam include impacts on businesses, stress, phishing, loss of corporate assets, inconvenience for the individual users, contribution to the carbon foot print of IT (or global warming), legal issues and exposure to viruses, spyware and Adware to name a few. Research shows that spam contributes more than 80% of e-mails with an increase in its growth rate every year. Spam is not limited to emails. It has started affecting other popular applications like VoIP, cellular telephony, traditional telephony, and instant messaging services. Spam in VoIP is more severe to spam in emails. This book also presents the various anti-spam approaches and discusses the advantages and the drawbacks of these approaches. Fighting spam with a combination of different anti-spam technological approaches gives better results.