Plant cells harbour two types of endosymbiotic organelles, notably mitochondria and plastids. As a consequence of the endosymbiotic gene transfer, the majority of their proteins is in both cases encoded in the nucleus and post-translationally "re"-imported into the respective target organelle. The corresponding targeting signals are in most instances specific for a single of these organelles. However, in recent years more and more proteins were identified which exhibit dual targeting properties, i.e. they can be transported into both mitochondria and plastids. In this thesis, the alpha-MPP2, one of the two isoforms of the substrate binding subunit of mitochondrial processing peptidase of Arabidopsis thaliana, was identified as a novel member of the class of nulear-encoded organelle proteins with dual targeting properties. As demonstrated by in organello transport experiments and by in vivo localisation studies, the N-terminal region of the alpha-MPP2 precursor comprises transport signals for the import into mitochondria as well as into chloroplasts. Using N-terminal as well as C-terminal truncations of the dual transit peptide of alpha-MPP2 we have demonstrated an unusual architecture of this transport signal, suggesting a composition of three functionally separated domains. Furthermore, a systematic screening of the model plant Arabidopsis thaliana was applied in this thesis to identify candidate proteins with potential dual targeting properties allowing a valuation of frequency and importance of this phenomenon. A selection of these candidates was analysed with regard to their transport behaviour by two complementary experimental approaches (in organello, in vivo). The experiments led to the identification of an unexpected high number of such proteins suggesting that the dual targeting phenomenon is considerably more widespread than usually anticipated.