This report provides an introduction to the key aspects of online advertising and highlights some of the main benefits and risks for consumers, drawing on behavioural insights as relevant. It was prepared by Anna Barker of the OECD Secretariat, initially to support discussion at the Roundtable on Online Advertising during the 95th session of the Committee on Consumer Policy (CCP) in April 2018. This paper was approved and declassified by the CCP by written procedure on 28 September 2018 and prepared for publication by the OECD Secretariat. This publication is a contribution to the OECD Going Digital project, which aims to provide policymakers with the tools they need to help their economies and societies prosper in an increasingly digital and data-driven world. For more information, visit www.oecd.org/going-digital.