This book examines the practices of cybercriminals who steal and sell personal information acquired through various means, including mass data breaches, to engage in cybercrime and fraud. Using data from multiple English and Russian language web forums, the authors identify the range of products sold in these active on-line marketplaces and the prospective profits earned by these actors. The social organization of these markets is analysed using sociological theory to understand the sophistication of the markets. Social network analyses of the relational networks of participants are also utilised to examine their sophistication and structure. In doing so, this work will contribute to the development of cybercrime studies, and will appeal to both social and computer scientists alike with an interest in the human aspects of cybercrime.