Analysis and interpretation of behavior-environment relations are increasingly being conducted with data that have been derived descriptively. This paper provides an overview of the logic that underlies a sequential analytic approach to the analysis of descriptive data. Several methods for quantifying sequential relations are reviewed along with their strengths and weaknesses. Data from descriptive analyses are used to illustrate key points. Issues germane to contingency analysis in natural environments are discussed briefly. It is concluded that the conceptual distinctions among contiguity, contingency, and dependency are critical if the logic of sequential analysis is to be extended successfully to a behavior-analytic account of reinforcement in natural environments. (Contains 4 figures.)