Aim of the present thesis was to assess self incompatibility status in olive cultivars Koroneiki, Kalamata, Mastoidis and Amigdalolia, to investigate their suitability as pollinators according to reciprocal cross-compatibilities and to determine the impact of abiotic factors on the sexual reproductive system of olive. Experiments were implemented in Oliviculture and Post Harvest Physiology - The Institute of Olive Tree and Subtropical Plants of Chania and Laboratory of Plant Biotechnology - Institute of Viticulture, Floriculture & Vegetable Crops of Heraklion, NAGREF, Laboratory of Plant Biotechnology - Foundation of Research and Technology of Heraklion (FORTH), Laboratory of Electronic Microscopy “Vasilis Galanopoulos” - School of Biology, University of Crete, during 2004 - 2009. In Koroneiki, cross-pollination and free-pollination significantly increased fruit set which averaged between 8,4-10 %. No significant differences were recorded between cross-pollination treatments, in three-year averages. Even the treatment of self-pollination resulted in high fruit set (4,9 %). In Kalamata three-year averages, the highest fruit set was recorded by free-pollination followed by Koroneiki. Mastoidis and Amigdalolia were less effective pollinators while the lowest fruit set was recorded after self pollination. In Mastoidis, free-pollination significantly improved fruit set while the other cultivars were similarly effective pollinators (3-year averages). Though, fruit set after pollination with pollen of Kalamata was similar to that of self pollination indicating potential cross-incompatibility. In the case of Amigdalolia, all cross-pollination treatments increased fruit set compared to self-pollination without significant differences between them. Though, fruit set after pollination with pollen of Mastoidis was similar to that of self pollination indicating potential cross-incompatibility. In the Solanaceae, Rosaceae and Scrophulariaceae families where the molecular mechanism of SI has been extensively studied, ribonucleases of the style (RNases) were found to be involved. On the contrary, in olive (Olea europaea L.) that also exhibits gametophytic SI, the molecular mechanism that controls the self/non-self recognition is unknown as in the Oleaceae family as a whole. In the present thesis, phylogenetic trees of identified S-RNases are presented and the similarity or diversity level of their aminoacid and nucleotide sequences are discussed in alignment with the botanical relationship between the plant species they were identified in. The conserved and the hypervariable regions of the S-RNases sequences are analysed through multiple alignment in order to select primers for PCR. In this paper a first approach to study the molecular mechanism of SI in olive is presented through bioinformatics analysis. Additionally, cDNAs expressed in pistils of Kalamata and Mastoidis were cloned. Sequencing and bioinformatics analysis revealed high similarity level with S-RNases of Rosaceae and Solanaceae which indicate that self-incompatibility mechanism in olive includes the metabolic pathways associated with S-RNases.