This chapter is in five parts. The first considers how the term ‘private investigation’ is to be defined. The second looks at the historical trajectory of private investigation in North America and Britain. Sections three and four, respectively, examine the structure and functions of the private investigative sector. Section five explores the issues of efficacy, ethics and regulation in respect of private investigation. The final section reflects back on issues raised previously in the chapter to consider the changing character of the public-private interface in security governance.