  The development of services and the growing demand for resources sharing
among users from different organizations with some level of affinity have
motivated the creation of Identity Management Systems. Identity Management has
gained significant attention in recent years in the form of several projects
producing many standards, prototypes and application models both in the
academia and the industry. However, the interoperability between different
Identity Management Solutions is still a complex challenge yet to achieve. The
user can only use one Identity Provider within a single Service Provider
session, when in many scenarios the user needs to provide attributes from
multiple Identity Providers. This paper presents the state of the art of our
researches and it focuses on two main topics: first, to provide a detailed
study about the Identity Management and the integrated disciplines and
technologies in general; secondly, to summarize the main approaches that have
been proposed to overcome the interoperability challenge.
