This research is based on an analysis of the covert investigation strategies adopted by the Metropolitan Police Hi-Tech Crime Unit, aiming to (i) assess the legislative and operational challenges and developments that shape the policing of online child sexual abuse; (ii) advance understanding of online child sexual abuse with respect to the downloading, production and distribution of indecent images of children; (iii) advance understanding of the grooming process; (iv) explore the reactive and proactive strategies for policing online child sexual abuse; (v) explore the online interaction between undercover police officers and sex offenders; and (vi) generate evidence-led policy recommendations for combating the problem of child sexual abuse online. This research is based on tactics used by the Metropolitan Hi-Tech Crime Unit, and is not representative of UK policing as a whole.