Thousands of definitions are in use today. They have legal, social service, medical, psychological, or sociological orientations, focusing on parental conduct, harm to the child, or both. These definitions often lack comparability, but the definition chosen can affect the results of a study. Other problems with the existing definitions are their imprecision and the lack of specification of the type of maltreatment involved in a particular study. Methods for improving the definitions should include a careful determination of definitional needs, the development of operational definitions to meet those needs, and the circumspect statement of findings based on the limitations imposed by the definitions. Thus, definitional issues should be an explicit part of research methodology.