The article begins by examining the variety of meanings that can be given to the word "theory", the different attitudes that may be taken towards theories of these various types and some of the problems associated with them. The second half of the article focuses on one of these types, explanatory theory, and the question of what is required if case study research is to be effective in producing sound theories of this sort. A range of important distinctions is outlined and questions are raised about the possibility of, and need for, such theories. (Contains 9 notes.)