Leafhoppers and planthoppers use either one of two feeding strategies: sheath feeding or lacerate and flush feeding. Electronic monitoring revealed that most sheath feeding leafhoppers ingest from the vascular tissues during sustained ingestion probes. Lacerate and flush feeders ingest from mesophyll parenchyma during short duration probes. Data from electronic monitoring suggest that leafhoppers and planthoppers are more flexible in feeding strategies than previously outlined by the Feeding Guild Concept. Although relatively few species have been electronically monitored, several demonstrate considerable overlap between the phloem, xylem, and mesophyll feeding guilds. Electronic monitoring as a tool in host plant resistance studies has been useful not only in demonstrating the decline in hopper feeding duration on resistant varieties, but also in elucidating behavioral changes from one activity to another on resistant plant cultivars. Vascular feeding hoppers react to resistant plants in a variety of ways, usually by reducing overall probing duration and/or by switching to another tissue for ingestion. Mesophyll feeding leafhoppers apparently do not reduce probing time, but switch to less damaging probing styles when electronically monitored on resistant varieties. Results from electronic monitoring studies suggest that some of these behavioral responses do not fit into the plant resistance categories of antibiosis, antixenosis, or tolerance and justify the broadening of these definitions or the erection of a fourth category which permits a nonrejection response on the part of the insect.