The grape breeding industry is dynamic, with new cultivars being released annually. In recent years there has been interest in breeding new grape cultivars to develop a new gene with wider ecological adaptation and to provide a response to viral pathogens. To achieve these goals, biotechnological approaches to plant disease are employed. Genomic studies and genetic markers can significantly speed up the selection of resistant seedlings. Natural virus resistance may be exploited by transfer of genes from resistant genotypes. Plant transformation has become an essential tool for plant molecular biologists, and genetically improved plants are the focus of many breeding programs. Integration of genes from diverse biological sources into grape genomes promises to broaden the gene pool and tailor plant varieties for specific traits. This chapter presents the molecular characterization and localization of T-DNA insertion in grapevines carrying genes of viral origin. Detailed analyses have revealed the crucial factors that influence transgene expression. Plants appear to respond to infection by activating a post-transcriptional gene silencing mechanism.