Forensic linguistics is at the cutting edge of the undercover policing of child sexual abuse on the open internet and dark web, and language and identity is a fundamental part of this. The authors have drawn on their extensive experience in training undercover officers to develop innovative methods in identifying the creation and performance of online personas, crucial in detecting identity disguise online. This groundbreaking book demonstrates these methods through case studies, whilst also exploring the link between language and identity. By bringing together previously opposed positions in forensic authorship analysis, the book develops a novel theory of linguistic identity, which will resonate not just in forensic authorship research but in sociolinguistics more widely. This unique forensic linguistic project has real-life impact in assisting the police in their investigation of online abusers, and has impact for students and researchers of linguistics, through its contribution to the research of linguistic identities.