Since the recognition that louping-ill, known for well over 100 years as an 
epizootic disease of sheep in Scotland, was caused by a virus transmitted by 
arthropods, many other arthropod-borne viruses capable of causing encephalitis 
in domestic animals or man have been discovered. The author reviews here the 
knowledge at present available on these viruses, originally termed 
"arthropod-borne encephalitides viruses" but now often referred to as "arbor 
viruses".In this discussion of the host and vector relationships of the two 
broad groups of arbor viruses - the mosquito-borne and the tick-borne-and of the 
distribution, epidemiology and control of the various diseases they cause, the 
author includes an outline of the types of investigation likely to provide the 
most useful information, stressing in this connexion the value of ecological 
surveys.