Online social media platforms are severely compromised by the existence of malicious actors such as bots on Twitter, vandals on Wikipedia, fake accounts on Facebook, trolls on Twitter and Slashdot, and spammers who seem to be omnipresent. This tutorial presents methods to identify malicious actors in at least 4 settings: Twitter, Facebook, Slashdot, and Wikipedia. We will look at 4 broad categories of methods: (i) network based techniques where the structure of the social network is used, (ii) text based methods where the linguistic content of posts is examined, (iii) behavior-based methods which study actions of users, and (iv) real-time processes which enable defenders of social media to keep a step ahead of malicious actors. The tutorial will identify commonly used features for classifying actors into malicious vs. benign and will give a brief explanation of different algorithms both specific to social platforms and general algorithms that are platform neutral.