The new high statistics deuterium data set of HERMES shows significant semi-inclusive single spin asymmetries of identified pions and kaons. The asymmetries observed on the deuterium target differ from those measured on the proton target. The appearance of non-zero single spin asymmetries can be explained in terms of a non-zero transversity distribution combined with a non-zero Collins fragmentation function. However, the detailed interpretation of the data is model dependent as it turns out that the asymmetries are dominated by higher-twist effects. HERMES data that are currently being taken on a transversely polarized target will allow for a model-independent extraction of the transversity distribution.

PACS numbers: 13.60.Hb, 13.88.+e