Phytophthora infestans is an Oomycete plant pathogen, and causes late blight disease on tomatoes and potatoes. This is arguably the most serious disease of these crops in temperate climates such as Britain’s. An SSR genotyping study was undertaken to assess the extent to which P. infestans populations on tomato and potato in Britain are host-specialised. No evidence of host specialisation was found, although very high levels of genetic diversity were found in the P. infestans populations sampled from gardens and allotments, suggesting that these settings could be a source of new P. infestans genotypes, possibly arising from sexual recombination. Additionally, field trials and associated marker-assisted selection work were carried out in conjunction with Burpee Europe Ltd., Pro-Veg Seeds Ltd., and the Sarvari Research Trust, with the aim of developing blight-resistant tomato cultivars for commercial release. A wide range of germplasm was screened, identifying promising material for breeding work. A line developed in this project, carrying the Ph-2 and Ph-3 genes for P. infestans resistance, has been released by Burpee Europe Ltd. under the name “Crimson Crush”. Additionally, detached leaflet studies were conducted to investigate how different resistance genes in combination interact with common P. infestans isolates. Findings included an indication of some residual effect of the “defeated” Ph-1 gene, the potential of some aggressive P. infestans isolates recovered from British gardens to overcome combined Ph-2 and Ph-3 resistance, and an insight into how the Ph- genes affect components of resistance (i.e., infection efficiency, latent period, rate of lesion expansion, and sporulation intensity), and may be complementary to each other. Finally, a genetic mapping study was carried out to investigate the genetic basis of resistance seen in tomato cultivar Koralik. Resistance QTLs were not reliably identified in Koralik, but the study did indicate the presence of resistance QTLs on Chromosomes 4 and 7 of the other parent in the cross, NC2-CELBR. Potentially useful QTLs affecting soluble solids content of fruit were identified in Koralik. Taken as a whole, the findings of these studies indicated that novel tomato cultivars with P. infestans resistance from a broad range of genes are needed to combat the threat from a highly diverse and evolving P. infestans population. The breeding and mapping work undertaken in this project makes some contribution to addressing this challenge, although further work is needed to fully capitalise on this.