This chapter examines the theoretical bases for forensic authorship analysis by considering a murder case where evidence was provided by Malcolm Coulthard. Jenny Nicholl had disappeared on 30 June 2005 and Coulthard’s linguistic analysis suggested that she was unlikely to have texted the final messages sent from her phone and that her estranged lover, David Hodgson, was one of a small group of possible authors. Hodgson was convicted of Jenny’s murder in 2008. Starting from this practical application I examine theories of authorship and of the linguistic individual from a cognitivist and a stylistic perspective and argue that, although cognitivist theories might be better at explaining within-author consistency, they struggle to explain between-author variation, whereas stylistic theories provide potentially convincing explanations for variation between authors but have less to offer when considering why an author might stay stylistically constant across time or social situations. I explore how debates about idiolect can be avoided by simply applying the ideas of consistency and distinctiveness and demonstrate a method through which this can be measured. I report how Coulthard’s approach was supported in Hodgson’s appeal hearing and how both the theoretical positions and the empirical methods have been further developed in subsequent work.