Neighborhood policy is invariably based upon some “theory of the problem.” Plainly, if that theory is inaccurate or incomplete, as we think current theories are, it may lead to “solutions” that are unavailing or worse. Currently, public intervention to arrest neighborhood decline assumes that it is premature or unnecessary. However, if as some persons feel, neighborhood decline — like the decline of living organisms is the inevitable consequence of advancing age, it is not a problem that can be prevented or corrected. Similarly, if neighborhood decline involves no more than an inevitable transfer of obsolescing structures to ever-lower income groups, one might view the phenomenon as simply an occurrence having neither positive nor negative connotations. The problem for policy in this situation would have to do, instead, with the distribution of income in society. Finally, to the extent that decline of individual neighborhoods is the consequence of rising living standards and the concomitant rejection of marginal segments of the housing inventory by those who can afford to maintain homes ingood-as-new condition, one can regard decline in a generally positive light. Depending on the situation, therefore, neighborhood policies either could seek to prevent and reverse decline or could focus on techniques that would help neighborhoods adjust downward and prevent the effects of decline from influencing surrounding areas. This essay examines the meaning and causes of neighborhood succession, and the relationships between succession and decline. Its purpose is to cut through the conceptual morass of definitions and assumptions about neighborhood change and in this way to clarify the “theory of the problem.” The essay has six substantive parts.