This chapter describes the concept of product authentication as a system which comprises the authentication device, tools to examine the device and examiners who are trained and equipped to carry out the examination of the device. It shows that the purpose of authentication systems is to detect counterfeit goods in the supply chain and thus prevent them from reaching consumers, not to prevent the production of counterfeits. Overt, covert and networked authentication features are described, and a conclusion reached that the most effective features combine overt and covert elements. The function of track and trace as an anti-counterfeiting method is discussed, concluding that it offers control of supply chains but does not provide authentication, and it can only operate in the legitimate supply chain, not in the illicit supply chain, which is the route used by many counterfeiters. The function of smartphones in authentication is examined, concluding that they have a role to play in the hands of specialist examiners but are unlikely to lead to the involvement of the public in examining goods to determine whether they are genuine or fake. Finally, the chapter discusses the return on investment for authentication systems.