The present thesis deals with the description of the fossil chelonians of Greece, mainly focusing on the detailed description of thousands individual specimens from more than 40 localities, including many new specimens and nearly all the previously published ones. Also, efforts have been made to place these taxa into a phylogenetic context, and to generate some hypotheses on the distribution, the role of chelonians in biostratigraphy and to explore their paleoecology through histological sections. The Greek paleochelonofauna is the most diverse and best preserved in the South Balkans with numerous testudinids (both small and large in size) and geoemydids. Also present, but to a smaller extent, are the emydids and trionychids, whereas a single pleurodire has been found. In general, the taxonomic diversity described in this thesis is significantly different than previously thought, since several taxa previously mentioned are found not valid, additional taxa have been identified, as well as the presence of at least three new taxa is proposed. Fossil chelonians are present in Greece since the Early Miocene, but their main geographical distribution is established since the Late Miocene. The evolution of several clades is in good accordance with the significant geotectonic and climatic changes that occurred in the circum Mediterranean area. As for the phylogenetic analysis, a new matrix of 81 morphological characters for the testudinids is defined, producing promising results that allow discussion of the phylogenetic relationships and evolutionary history of the major testudinid clades, as well as to investigate the evolution of gigantism in this group. The Greek fossil chelonian assemblage, standing between Eurasia and Africa, is of critical importance for the evolution of various chelonian clades from the beginning of the Neogene and onwards. Finally, as it is evident from the present thesis, the study of fossilized chelonians could provide a significant advance on our knowledge of paleoecology and biostratigraphy of Greece.