Abstract: In this paper we propose a taxonomy for image authentication techniques that takes into account three different features: the level of integrity verification, the approach to the generation of the authenticator and the capability of localizing manipulated areas. The goal is to revise the current and very heterogeneous bibliography on this topic, and to define a set of basic requirements for an authentication service, in order to identify which ones have already been met and which ones remain challenging. The major algorithms proposed in the last decade are examined according to the classification criteria; advantages and drawbacks of the possible approaches are reported. In particular we investigate the relationship between the well established digital signature techniques and the emerging watermarking approaches. As a result, we propose a methodological approach to the design of robust, secure and efficient authentication algorithms.