A major turning point in the study of metazoan evolution was the recognition of the existence of the Ecdysozoa in 1997. This is a group of eight animal phyla (Nematoda, Nematomorpha, Loricifera, Kinorhyncha, Priapulida, Tardigrada, Onychophora and Arthropoda). Ecdysozoa is the most specious clade of animals to ever exist and the relationships among its eight phyla are still heatedly debated. Similarly also the relationships among the three sub-phyla (Chelicerata, Pancrustacea and Myriapoda) within the most important ecdysozoan phylum (the Arthropoda) are still debated. Indeed, the two major problems in ecdysozoan phylogeny refer to the relationships of Myriapoda within Arthropoda, and of Tardigrada within Ecdysozoa. Difficulties in ecdysozoan relationships resides in lineages characterized by rapid, deep divergences and subsequently long periods of divergent evolution. Phylogenetic signal to resolve the relationships of these lineages is diluted, increasing the likelihood of recovery of phylogenetic artifacts. In an attempt to resolve the relationships within Ecdysozoa, consilience of three independent phylogenetic data sets was investigated. EST and rRNA and microRNA (miRNA) data were sampled across all major ecdysozoan phyla. In particular, a major contribution of this thesis is the first time sequencing of miRNAs for all the panarthropod phyla. MicroRNAs are genome regulatory elements that recently emerged as a source of useful phylogenetic data (Sempere et al. 2006) because of their low homoplasy levels. The considered data sets were analysed under phylogenetic methods and models, implemented to minimize the occurrence of phylogenetic reconstruction artifacts to understand the evolution of Ecdysozoa. Analyses of independent data types recovered well supported and corroborating evidence for the monophyly of Panarthropoda (Arthropoda, Onychophora and Tardigrada), a sister group relationships between Myriapoda and Pancrustacea within Arthropoda, and the paraphyly of Cycloneuralia (Nematoda, Nematomorpha, Loricifera, Kinorhyncha and Priapulida). !