WE SHALL discuss certain generalizations regarding principles and practices which rationalize efforts to eradicate communicable diseases, vis-avis those aimed at their control. While the unqualified word "eradication" may be understood to signify the world-wide extermination of a biologic species, e.g., of an agent or a vector or a reservoir of human disease, our use of the expression, disease eradication, is restricted to its application on a state, regional, island-wide, continental, or on any substantial area or population basis, so long as its extent is clearly delineated. Our understanding and use of the terms "eradication" and "control" of endemic disease is one used previously' and is as follows: "Control is the purposeful reduction of specific disease prevalence to relatively low levels of occurrence, though transmission occurs frequently enough to prevent its permanent disappearance; eradication, ditto but to the point of continued absence of transmission within a specified area." Mathematically, the approach to either of these objectives is under way when the general trend in the evolution of a particular disease in a specified population area is for successive crops of new cases to be generally less numerous than preceding ones, that is, on the average, each new case is succeeded by less than one new case. As long as this relationship prevails, the specific morbidity trend will be downward, and the negative slope must ultimately approach the baseline. If this reductive process continues until the number of newly transmitted cases reaches and remains at zero, eradication of the disease has been achieved. If, on the other hand, the decline in new cases is halted by circumstances which slow it down to a fluctuating equilibrium at some point approaching but not quite reaching zero, the disease may be declared administratively to be under control, though it is certainly not eliminated. Thus control is a more relative and less absolute term than eradication.