This open access book is the first comprehensive guide to identifying antisemitism online today, in both its explicit and implicit (or coded) forms. Developed through years of on-the-ground analysis of over 100,000 authentic comments posted by social media users in the UK, France, Germany and beyond, the book introduces and explains the central historical, conceptual and linguistic-semiotic elements of 46 antisemitic concepts, stereotypes and speech acts. The guide was assembled by researchers working on the Decoding Antisemitism project at the Centre for Research on Antisemitism at Technische Universität Berlin, building on existing basic definitions of antisemitism, and drawing on expertise in various fields. Using authentic examples taken from social media over the past four years, it sets out a pioneering step-by-step approach to identifying and categorising antisemitic content, providing guidance on how to recognise a statement as antisemitic or not. This book will be an invaluable tool through which researchers, students, practitioners and social media moderators can learn to recognise contemporary antisemitism online - and the structural aspects of hate speech more generally - in all its breadth and diversity.