During infection of plant tissue by Agrobacterium, a fragment of Ti plasmid (T-DNA) is transferred to a cell. T-DNA is delimited by 25 bp direct repeats, which are necessary to direct it. Any piece of DNA flanked by these sequences can be transferred to the plant cell, where it becomes integrated into the plant genome. The processing and transfer of T-DNA are mediated by vir region localized on the Ti-plasmid. Based on the summarized model of plant transformation via Agrobacterium, the key factors involved in this mechanism are reviewed. There are two basic components of the process: Agro strain with its vector system and recipient, usually totipotent plant cell. The most important factors, which should be taken into consideration, include: Agrobacterium host specificity and its vector system, tissue culture techniques and their limitations, chemical agents inducing virulence genes and selection strategies. Some of them can influence the expression pattern of the introduced genes.