Investigations of plant mitochondrial protein import have been very limited and few reports exist in the literature. The work presented in this thesis describes the development of an in vitro Zea mays mitochondrial protein import system, which has enabled the import of plant nuclear-encoded mitochondrial proteins to be examined. This Z.mays import system was characterised and partially optimised with the Nicotiana plumbaginifolia manganese superoxide dismutase (MnSOD). Results suggest that the energy requirements for protein import into plant mitochondria are similar to those of Saccharomyces cerevisiae and Neurospora crassa, requiring both an energised inner mitochondrial membrane and ATP. The inclusion of 1,10-phenanthroline inhibited the processing, but not the import of MnSOD and indicated that the processing activity within Z.mays mitochondria was dependent upon the presence of metal ions. The plant mitochondrial processing protease may therefore be similar to the characterised S.cerevisiae and N.crassa matrix processing protease. The import of the Z.mays adenine nucleotide translocator (ANT) protein was then investigated. Unlike the ANT of S.cerevisiae and N. crassa, this plant ANT was synthesised as a precursor protein, which was processed upon import into mitochondria isolated from both Z.mays and Solanum tuberosum. The subsequent isolation of an S.tuberosum ANT cDNA clone PANT-1, enabled the import of a second plant ANT to be investigated. Results corroborated the findings previously obtained with the Z.mays ANT and it was therefore concluded that the import of plant ANT proteins are distinctly different from those of S.cerevisiae and N.crassa.