Numerous bacteria have been isolated from within plants, and many reported from xylem, but only three species of xylem-limited bacteria (XLB) that are fastidious in cultural requirements, are plant pathogens, and exclusively occupy xylem, have been well characterized. Two XLB, Xylella fastidiosa and Pseudomonas syzygii, are transmitted by sucking insects that feed on xylem sap but are not transmitted mechanically from plant to plant. In contrast, Clavibacter xyli is mechanically transmitted to plants by cutting tools. All of these XLB occupy a highly specialized yet diverse ecological niche: the water-conducting systems of an extremely wide range of plant hosts. A variety of detection methods are available as diagnostic aids; each method has advantages and disadvantages; no single method is best for all uses. Molecular and genetic comparisons of strains of XLB lag behind progress being made for many other plant-pathogenic bacteria, but such studies are needed to answer important questions: (a) How do XLB move from cell to cell within plants? (b) What are the physiological and genetic bases of plant host specificity for XLB? (c) Why are only xylem-feeding specialists vectors of X. fastidiosa (and probably P. syzygii), when many leafhoppers feed regularly (but not continuously) on xylem?