This chapter reviews the different experimental possibilities of delivering genes to recipient plants. Transient transformation is another approach of gene transfer that is frequently used among higher plants. The choice of the transformation system clearly depends on the plant species with which one is dealing. In contrast to stable transformation, assays for transient gene expression allow the very rapid evaluation of gene expression of a given construct within days. The instrument of gene transfer has been very rapidly introduced into the arsenal of general methods of basic plant research where it opened up whole new avenues of research. Plant viruses have always been very attractive for plant genetic engineering as potential transmitters of foreign genes. Protoplasts have also been the first system for demonstrating transient gene expression in higher plants. In order to test a given gene construct for transient expression, protoplasts have to be isolated in large amounts and should be uniform and vital.