Body size and species identity both contribute to the complex webs of interaction that determine the structure and function of ecosystems. The SIZEMIC Research Networking Programme will attempt a synthesis of size and species-based approaches for describing structure and energy flux in ecosystems and seek to understand how the properties of individuals lead to observed patterns of size structure and diversity. This synthesis, building on recent theoretical developments in aquatic and terrestrial ecology, will be used to develop and test size-based models that might be used to assess and monitor the impacts of human activities on ecosystems. SIZEMIC will provide a focus for collaboration between theoretical and applied ecologists working on terrestrial and aquatic ecosystems and provide opportunities for young European scientists to work across existing research boundaries.