The identification of causal variables is a central component in the design of many treatment programs. This article reviews the underlying principles and methods of identifying important causal variables for clients in pretreatment psychological assessment. The article defines causal and functional relationships, and reviews methods of deriving causal inferences about clients' behavior problems. These include rational derivation, causally focused self-report instruments, causal marker variables, time-series assessment, and manipulation strategies. Measuring and inferring causal relationships are constrained by multiple modes and parameters of behavior disorders, the bidirectional and nonlinear nature of causal relationships, the dynamic and unstable nature of causal relationships, and the causal properties of change in a causal variable