Agroinfection, the delivery of viral or viroidal sequences to plants by Agrobacterium, can be used to approach important basic questions in plant molecular biology. The combined use of three biological entities allows the analysis of plant-Agrobacterium interactions using the virus as a marker for T-DNA transfer, or the investigation of viral biology using Agrobacterium as a delivery vehicle for the virus. Plants transgenic for viral constructs offer possibilities for studying recombination, plant protection, and development of high copy number plant vectors. Relevant examples of these approaches are discussed.