Assessment of a patient's functional ability to perform activities of daily living is an essential part of nursing. Measuring the functional ability of older adults is a potent predictor of a patient's morbidity and hospital outcome. The information generated from assessment scales is only constructive if the information generated is clinically useful and scientifically reliable. The Katz activities of daily living and the Barthel Index are two of the oldest competing indices for assessing activities of daily living. The Katz activities of daily living and the Barthel Index evaluate a patient's function in terms of level of independence or dependence when performing certain activities required for daily living. This paper will compare their significance and usefulness to nursing practice of the older adult. As information is gathered and interpreted to complete these assessment scales, the relative merits in specific situations of these two oldest competing indices need to be considered to ensure their appropriateness and sensitivity. Often, with the use of assessment scales for activities of daily living, the results often tend to be subjective and lack a systematic approach. Subjective assessment of patients makes it difficult to evaluate the effectiveness of treatments, leading to bias and the misinterpretation of data. This may be critical in the care of older adults, where presenting problems are often complex and multi-dimensional. As assessment of functional ability has become more important to nursing practice, this paper will investigate the Katz activities of daily living and the Barthel Index for reliability and explore the variations that exist between the two indices. Although research on assessment scales has been ongoing since the work of Katz et al. (1963) there remains conceptual issues as to what constitutes activities of daily living and defining terminology associated with the measurement.