Theoretical studies about citizenship have had a resurgence in recent years. Unfortunately, in this area of study there has been a lack of clarity and precision. The reasons are many, not only methodological. In general, reflections on citizenship in the juridical field have been made from a non-positivist and metaethically cognitivist approach. That means that the attribution of rights and duties as well as the access to and loss of citizenship have an ethical foundation. In addition, reflections on citizenship have been restricted to a particular political and ethical context, preventing the construction of a general theory of citizenship My goal with this lecture is to reflect on the functional theory of citizenship, to expose its most important arguments and question them, not from an alternative theory, but with the aim of submitting much of its thesis to analysis and rethinking it in order to obtain a better description of citizenship phenomena.