This thesis looks at the system of verbal stems/diatheses/binyanim in the Semitic languages Akkadian, Gәᶜәz, Amharic, Arabic, Cairene Arabic, Hebrew, Aramaic, and Phoenician-Punic. The functions each of the stems can convey (such as the passive, reflexive, causative, factitive etc.) are surveyed and an attempt to reveal an underlying principle that can unite the various functions is made. A theory of prototypical transitivity, i.e. transitivity as a semantic rather than syntactic concept defined by means of a prototype, has been applied and has proven a useful tool in analyzing the verbal stems, and the three parameters of classification within this approach (volition, instigation, and affectedness) have turned out to be powerful means of distinguishing minute nuances between stems. Initially the formation of verbal predicates in general, and the way participants are associated with them, is discussed. These general sections show that a theory of the binyanim that can account equally well for isolated forms as for rich and intricate interdependencies between several stems formed from the same root is possible if we look for the basic mechanisms that the binyanim encode. Through the analysis it is revealed that these basic mechanisms are for instance the affectedness of the subject, the lack of distinguishability of participants, and the focus on an instigating participant. It is also found that these basic properties of participants can receive a special focus, and that such focus shifts are another basic function of the binyanim. The difference between a causative and a factitive clause can for example be identified as the former’s focus on the cause’s instigation and the latter’s focus on the causee’s affectedness.