The work presented in this thesis uses proteomics and biochemical analyses to identify and investigate potential pathogenicity factors in Phytophthora infestans. Proteomics was also used to analyse the P. infestans-tomato (Lycopersicon esculentum) interaction.  2DGE was used to identify interaction specific proteins in intercellular wash fluid (IWF) from tomato leaves infected with P. infestans.  Protein profiles of IWF from infected leaves were compared with profiles of IWF from healthy leaves.  Interaction specific proteins were identified by MALDI-T of MS and PMF.  Seven proteins of tomato origin were identified.  Most proteins identified belong to pathogenesis-related (PR) protein families. This thesis also describes characterisation of an actin depolymerising factor termed PiADF1.  PiADF1 was previously identified from mycelium culture filtrate separated by 2DGE.  The actin cytoskeleton plays an important role in plant defence responses.  It is proposed that PiADF1 may function in suppressing actin related host defence responses during the P. infestans-plant interaction.  Secretion inhibition assays were carried out which support the hypothesis that PiADF1 is secreted by P. infestans despite lacking a classical N-terminal signal sequence.  Gene expression analysis, fluorescence localisation in planta and phylogenetic analyses were also carried out.  Possible roles for PiADF1 in suppression of host defence responses are discussed. A potential host cell targeting (HCT) peptide motif, RxLR was recently discovered in extracellular proteins of biotrophic oomycetes.  This thesis describes initial attempts to identify proteins, which may be involved in RxLR-mediated HCT.  Proteins which may form HCT machinery are speculated upon.