The epidemiologic concept of tick-transmitted diseases has increased in importance with the recognition of the emerging infectious diseases, Lyme borreliosis, human monocytotropic and granulocytotropic ehrlichioses, and three different babesioses. Effective public health control of these diseases would depend upon critical knowledge of the vector biology of the ticks that transmit them. Rocky Mountain spotted fever and the human ehrlichioses are life-threatening yet treatable diseases. A major problem remains establishment of the diagnosis when treatment decisions are being made. Clinical manifestations, other than erythema migrans for Lyme borreliosis, do not provide strong diagnostic clues. Ehrlichiae or babesiae are often not detected in peripheral blood smears. Frequently there are no antibodies to these diverse agents at the time of presentation, and isolation does not yield sensitive and timely results. Polymerase chain reaction, still a research tool, promises the greatest sensitivity, specificity, and timeliness. Prevention by vaccines is not yet a reality, although OspA-based vaccines offer hope for the prevention of Lyme disease.