Assessing medical causality requires a clear understanding and discussion of causal relationships, and definitions of impairment may have special meaning in a legal context beyond their usual meanings in medical communications. In addition to assessing cause and effect and medical possibility vs medical probability, evaluating physicians must understand and carefully use terminology such as aggravation or exacerbation that have specific definitions and uses. Documentation of aggravation or causation depends on the acquisition, review, and analysis of medical documentation, including medical office and hospital records; available past medical records; results of tests or diagnostic procedures; results of permanent occupational or environmental surveys; medical information showing that an alleged factor could have caused or contributed to the effect; and occupational documentation of an alleged exposure or cause. Pitfalls during the assessments include the patient's aging, natural inherent limitations, self-abuse, and psychosocial factors that individually or collectively may confound medical causality and apportionment assessment. The evaluating physician alone is responsible for apportioning these factors and must provide the medical basis for all conclusions and opinions. Previous impairment of an organ system may be subtracted from the current impairment rating, and the written analysis should address separately a divergence between changes in impairment and disability.