Construction of differential characteristics in ARX designs application to skein. In this paper, we study differential attacks against ARX schemes. We build upon the generalized characteristics of De Canni`ere and Rechberger and the multi-bit constraints of Leurent.par Our main result is an algorithm to build complex non-linear differential characteristics for ARX constructions, that we applied to reduced versions of the hash function Skein. We present several characteristics for use in various attack scenarios: on the one hand we show attacks with a relatively low complexity, in relatively strong settings; and on the other hand weaker distinguishers reaching more rounds. Our most notable results are practical free-start and semi-free-start collision attacks for 20 rounds and 12 rounds of Skein-256, respectively. Since the full version of Skein-256 has 72 rounds, this result confirms the large security margin of the design.par These results are some of the first examples of complex differential trails built for pure ARX designs. We believe this is an important work to assess the security those functions against differential cryptanalysis. Our tools are publicly available from the ARXtools webpage.