In agent literature, a partially unexplored area is related to the integration of ever-wider opportunities offered by technologies such as Mixed Reality (MR) and Augmented Reality (AR). In this paper we present a framework called Augmented Worlds (AW), which provides a model and a technological support to develop a broad spectrum of agent-based AR/MR systems. Distinguishing key features of the approach include: bi-directional augmentation, support for existing cognitive agent technologies, support for developing open multi-user environments. In the paper, we describe first the conceptual model on which the framework is based, and then a concrete architecture and prototype implementation. Two case studies about real-world applications – an augmented museum and an augmented harbour – engineered with the framework are finally discussed.