INTRODUCTION Vectors are organisms that can introduce a pathogen such as a bacterium or virus into a plant to cause an infection. Insects, mites, and nematode vectors focus the movement of plant pathogens among immobile plants. Many insects or other arthropods may contain plant pathogens but cannot transmit these to plants and thus are not vectors. Some of our most important plant diseases require mobile vectors. Almost all plant viruses and all wall-free, plant pathogenic bacteria. known as mollicutes have recognized or suspected vectors. See elsewhere for insect vector transmission of bacterial plant pathogens. Much research on vector transmission seeks to understand the transmission process so as to explain why only certain kinds of insects or mites can serve as vectors and to identify what factors are required for transmission. Because insecticides applied to kill vectors frequently fail to control the diseases caused by the vector-borne pathogens, a good understanding of the relationships between vectors and the pathogens that they transmit is important.