Hemodialysis treatments yielding inadequate amounts of dialysis, as defined by urea kinetic modeling, are partially responsible for considerable mortality and morbidity in the United States. In almost 50% of dialysis treatments resulting in a Kt/V of < 1.0, the culprit is impaired delivery of the prescribed amount of dialysis. The factors involved in impaired delivery of dialysis are many and often elusive. If present and widespread, a search for the cause of the problem entails careful examination of the equipment and nursing procedures. If impaired delivery is a sporadic and infrequent event, a patient-specific investigation should be undertaken. In either circumstance, a clear understanding of the principles and practical aspects of hemodialysis greatly assists the nephrologist as a sleuth.