**1. Introduction**

In addition to their outstanding ecological importance, bryophytes have found numerous economic uses. On particular species, even has found applications for the production of recombinant proteins [1–3]. This has instigated the exploration of the potential for post-translational modifications in mosses in general and that of glycosylation in *Physcomitrella patens* (Hedw.) Bruch & Schimp in particular. At first, this moss appeared as a perfectly normal plant, as will be detailed in the next chapter. Only recently was it found that mosses in general have the ability to equip their proteins with features that have not yet been described elsewhere. In the last chapter, the differences between mammalian and moss O-glycosylation will be portrayed.
