This chapter aims at offering an overview of the properties that define prefixes as a class and, at the same time, at acknowledging the differences that emerge among the elements classed as prefixes. To this end, their function and semantics are explored, as well as their structural behaviour. On the basis of the distinction between internal and external prefixes, it is hypothesized that prefixes, as a heterogeneous class, cannot be uniformly analysed as adjuncts or as heads. It is concluded that what unifies prefixes as a class is their relational nature.