Computational trust is a central paradigm in today's Internet as our modern society is increasingly relying upon online transactions and social net- works. This is indeed leading to the introduction of various trust management systems and associated trust models, which are customized according to their target applications. However, the heterogeneity of trust models prevents exploiting the trust knowledge acquired in one context in another context although this would be beneficial for the digital, ever-connected environment. This is such an issue that this paper addresses by introducing an approach to achieve interoperability between heterogeneous trust management systems. Specifically, we define a trust meta-model that allows the rigorous specification of trust models as well as their composition. The resulting composite trust models enable heterogeneous trust management systems to interoperate transparently through mediators.