The following paper by Gian Luca Cattani and Peter Sewell Models for Name-Passing Processes: Interleaving and Causal is available as Technical Report 505 of Cambridge University Computer Laboratory electronically from http://www.cl.cam.ac.uk/users/glc25/modnppictr.html or http://www.cl.cam.ac.uk/users/pes20/ or, in hard copy, by emailing "tech-reports@cl.cam.ac.uk". An abstract is attached below. Luca Abstract: We study syntax-free models for name-passing processes. For interleaving semantics, we identify the indexing structure required of an early labelled transition system to support the usual pi-calculus operations, defining Indexed Labelled Transition Systems. For non-interleaving causal semantics we define Indexed Labelled Asynchronus Transition Systems, smoothly generalizing both our interleaving model and the standard Asynchronous Transition Systems model for CCS-like calculi. In each case we relate a denotational semantics to an operational view, for bisimulation and causal bisimulation respectively. We establish completeness properties of, and adjunctions between, categories of the two models. Alternative indexing structures and possible applications are also discussed. These are first steps towards a uniform understanding of the semantics and operations of name-passing calculi.