ntroduction: Tachinidae is a diverse and important group of dipteran parasitoids. The very recent and rapid radiation of the family has posed problems for taxonomic classification and for understanding evolutionary relationships among taxa and patterns of host use.

Methods: Here we present a molecular phylogenetic analysis of the family employing more than 350 exemplar taxa and 4 nuclear genes (ca. 6 Kb).

Results/Conclusion: Maximum Likelihood and Bayesian analyses largely support modern classification schemes based on morphology and recent phylogenetic analyses, with some important exceptions. In particular, the subfamily Tachininae is both poly- and paraphyletic with one smaller lineage reconstructed as sister to all remaining tachinids, and the other main lineage also containing the subfamily Exoristinae. In general, relationships among taxa are well resolved, however, a number of relationships within the Dexiinae and between the Dexiinae and Phasiinae remain contentious. Evolutionary reconstruction of host associations indicates that, although some large clades exhibit host conservatism (e.g. Phasiinae), transitions among host orders are relatively frequent. Basal lineages tend to be associated with hemimetabolous or adult holometabolous host taxa and transitions to larval Holometabola, particularly within Lepidoptera, are associated with elevated rates of diversification.