The overall aim of this thesis was, to evaluate the potential of molecular markers, specifically SNPs and microsatellites, within DUS (Distinct, Uniform and Stable) testing of Lolium species. In the current DUS testing system new candidate cultivars are compared to all existing cultivars in common knowledge which is very labour intensive and expensive. As candidates pass, they become part of the reference collection. The reference collection is reaching an unmanageable size; therefore alternative testing methods are being sought. Initially morphological data for 23 cultivars comprising of Lolium perenne, L. mutiflorum, L. hybridum and Festulolium was analysed to define the relationships between them. The species grouped together, with the hybrids linking with the Italian ryegrasses. The exception to this rule was the hybrid Foyle, which linked more closely with the perennial ryegrasses. Another exception was the perennial ryegrass Bealey that linked more closely with the Italians. These groupings were then used to evaluate how successful the molecular markers were at replicating the same relationships. Initially, cytoplasmic diversity was analysed using Single Nucleotide Polymorphisms (SNPs). SNPs were ascertained from individuals within the sample set of cultivars. Screening for these SNPs highlighted species separation, but individual cultivars could not be identified. Again, Bealey linked with the Italian ryegrass cultivars. Patterns of cytoplasmic diversity were then analysed using microsatellites. Again, the split between L. perenne and L. multiflorum was marked, with Bealey as an exception. Further investigation into the cytoplasmic diversity was carried out, specifically the 'potential for identifying Essentially Derived Varieties (EDVs). Use of markers in this way relies on a low mutation rate over the loci, which was not the case in this study. The findings of the study are discussed with respect to the utility of cytoplasmic markers as a complement to the DUS testing system, and future directions suggested.