The term “agroinfection” was first used (1 ) to describe the use of Agrobacterium for the introduction of infectious molecules to plants. This implies infection of the host plant with a molecule, the “infectious agent,” generally a virus or viroid, that has the ability to replicate and spread within the plant; the introduction of parts of agents that do not have this potential is thus outside the scope of this chapter. Subsequently, the term “agroinoculation” has also been used by some authors to describe the inoculation step (2 ). Replication of the agent within the plant often leads to systemic viral or viroidal symptoms that witness T-DNA transfer, independently of T-DNA integration. Nontumorigenic strains of Agrobacterium may be used; this provides the additional possibility of regenerating transgenic plants containing all or part of the agent genome integrated in the plant nuclear DNA.