Orthotic and prosthetic clinicians make clinical decisions daily that affect their patients’ lives and the resources of the health-care system. These decisions are largely based on information gained taking clinical measurements of patient characteristics. Each measurement can be evaluated in terms of its reliability, validity, responsiveness and usefulness in the clinic. Clinicians and researchers studying clinical phenomena must have an understanding of these qualities of clinical measures and the sources of error that can make their data less useful. Measurement errors occur in all systematic investigations of natural phenomena. Neither the advanced state of our instrumented technology nor precision in our procedures can prevent random errors. Thus, it is important that orthotists and prosthetists familiarize themselves with the characteristics and occurrences of measurement errors in practice and research. This understanding will provide the basis to evaluate the impact of error and bias on measures and improve the clinical decisions made when reading or conducting clinical research. © 1995 Journal of Prosthetics and Orthotics All Rights Reserved.