Opportunity structures for crime create distinct patterns. These "signatures" draw attention to crime patterns that are not simply spatial distribution, although the spatial distribution of crime is an important part of many crime signatures. Both academic research and crime analysis uses crime signatures in analyzing how crime concentrations arise and in designing interventions that will reduce crime. This analysis involves opportunity structures that facilitate particular crime signatures. In changing opportunity structures through effective interventions, changes in the targeted crime signature should occur. The collection and analysis of data on the targeted crime signature should determine whether a crime prevention intervention has worked. Such information improves the internal validity of evaluations by eliminating rival hypotheses that evaluation designs may not be able to eliminate on their own. This chapter developed an evaluation procedure called SCEMA: Signature Characterization, Expectation, Measurement, and Analysis that applies this argument. SCEMA is not a substitute for standard evaluation designs, but a supplement to them. The chapter discusses 10 propositions that pertain to opportunity structures, signature, and signature change. This is followed by two examples of how signature changes can be used to help determine whether a situational-crime-prevention intervention is effective. This is followed by a description of the SCEMA procedure. The chapter's final section examines several critical issues raised by the use of crime prevention evaluations that use crime signatures as part of the analysis. 2 figures, 1 table, 2 notes, and 22 references