Causal assessments bring together information to reach conclusions about how biological effects happened or can be produced. In environmental studies, a causal assessment may be prompted by observations of undesirable biological effects, concerns over a source or stressor, or questions about the efficacy of management actions. Observational studies are often the principal source of evidence available for causal assessments of environmental issues, but are problematic because causes are not manipulated, replicated, or randomly applied. Pragmatic strategies for interpreting observational study results for causal assessment include breaking up and analyzing causal pathways by mechanistic segment; comparing evidence for alternative causal explanations; using study design or statistical techniques to isolate the effects of individual variables; modeling the effects of multiple variables together; and supplementing observational results with evidence of sufficiency, exposure, and symptoms. Weight-of-evidence approaches are well suited to causal assessments because they can provide a common framework for synthesizing disparate types of information while retaining the evidence supporting or refuting conclusions.