A major consideration in the use of genetical resistance against infection of crop plants is the ability of the pathogen concerned to evolve genotypes with virulence to overcome such resistance. This applies particularly to oligogenic forms of resistance with major effects and it is Dossible that forms of partial resistance may be durable and have economic benefit. This study was concerned with the development of screening methods and identifying possible sources of partial resistance in barley to two fungal foliar pathogens, Evysiphe gvaminis f.sp. hovdei3 causal agent of mildew and Rhynahospovium seoalis causing leaf blotch. With respect to barley mildew, lines from Ethiopia, Turkey and Israel, as well as lines and cultivars from two European Barley Disease Nurseries, were found in preliminary work to exhibit a wide range of response when exposed to natural inocula of E. gvaminis. A screening procedure was adopted to favour the selection of virulences, from the pathogen population, for particular host genotypes and indicated those lines which gave consistently low disease levels. When these lines were tested in conjunction with commercial cultivars against known isolates of E. gvaminis with various virulence combinations, different patterns of resistance response were evidenced. Firstly, vertical resistance was associated with commercial cultivars but not lines; secondly, consistently high resistance was shown by some lines indicating resistance factors other than those apparently present in most existing commercial cultivars and thirdly, some lines consistently showed intermediate levels of horizontal resistance. In tests on commercial cultivars grouped according to their barley mildew resistance categories, both intra- and inter-group differences were recorded. Variations in group characteristics between years was attributable to changing virulence combinations in the pathogen population. Variations within groups were low and inconsistent between assessments: in some cases adult plant resistance may have been important. The reported tolerance of Proctor may be associated with delayed infection of emerging leaves and little necrosis resulting from infection. Microscopic assessments indicated that leaf position and plant age may influence fungal development, although there were no apparent qualitative differences in pathogen behaviour on cultivars evidencing varying degrees of partial resistance. Studies on R. secalis were hampered by difficulties in ensuring epidemic development in screening tests, although the development of a system, based on automatic misting equipment eventually overcame these and susceptible cultivars became rapidly infected. In glasshouse trials on a range of lines and cultivars, infection above a threshold level did not lead to increased impairment of leaf functioning: growth habit may be of some importance in determining cultivar susceptibility to this disease. It was demonstrated that lesion development and level of spore production for different cultivars may not be correlated, the level of spore production being epidemiologically significant in the field.