To facilitate communication, people uses virtual meeting environment in everyday lives with the help of global online social networks (OSNs) in recent years. These networks help users to find novel friends and buildup their links worldwide. Moreover, sharing of information and thoughts are important features of OSNs. Users can share photos, recent activities, videos, interests, applications and much more with the help of the OSNs. The rate of using OSNs has increased rapidly in the recent years. OSNs like Google+, Facebook, Twitter, LinkedIn, Sina Weibo, VKontakte, and Mixi have become the preferred way of communication for billions of daily active users. Users spend maximum time to update their contents, communicate with early users and browse other’s account to find certain information which is the main implication of social network websites. OSN can eliminate the geographical and economical barriers between the users for the communication and information sharing. Due to this, it has become a fascinating test bed for numerous cyber-attacks comprising Cross-Site Scripting, SQL injection, DDoS, phishing, spamming, fake profile, spammer etc. This book entitled Online Social Networks Security: Principles, Algorithm, Applications, and Perspectives elaborates on various attacks with  classifications, countermeasures and consequences. It also highlights some key contributions related to current defensive approaches of various attacks on OSNs. Moreover, it represents machine learning and deep learning-based approaches to militate against various attacks on OSNs. Moreover, different technological solutions are also discussed that have been proposed so far against OSNs based attacks. The contributions of the topics, methodologies and results included in this book will educate readers towards the importance of incentives in any technical solution to handle attacks against OSNs. The best practices and guidelines will showcase how to implement various attack mitigation methodologies.