Nowadays, due to the enormous amount of (potentially sensitive) information available on-line as well as the large number of users world-wide accessing and publishing such data, guaranteeing security and managing trust have become crucial within the development and exploitation of systems. Open environments such as the Web do not assume that users are known in advance. In contrary, entities that never had an interaction in common may interoperate, and in such new transactions, traditional techniques do not apply. This book addresses the problem of security and trust management in open environments by using expressive policy languages associated to advanced mechanisms for the exchange, evaluation and enforcement of the policies specified with them. The two languages presented are the result of an evolution of the work performed within my PhD thesis. Overall, these languages provide a system with, among others, the ability to interpret, evaluate and enforce specified policies, exchange them with other systems, negotiate with other parties and even explain in natural language the policies being applied as well as the reason why a request was granted or denied.