Bioimpedance analysis (BIA) derives two main pieces of information--total tissue fluid content, which when referring to the whole patient is equivalent to the total body water (TBW), and cell mass, which in the limbs mainly reflects muscle. The relationship between these measures, expressed in different ways, is abnormal in dialysis patients due to muscle wasting combined with tissue overhydration. In both dialysis modalities this is associated with aging, comorbidity, and inflammation, and there is a conflict between achieving euvolemia to improve blood pressure control and prevent left ventricular hypertrophy on one hand, but risking episodes of hypovolemia and loss of residual renal function on the other. In peritoneal dialysis, the situation is exacerbated by hypoalbuminemia, whereas in hemodialysis BIA is unable to distinguish between the plasma volume and tissue edema components of interdialytic weight gain. In longitudinal studies BIA can identify changes in hydration following a defined intervention, and spontaneous loss in TBW consequent on muscle wasting not appreciated clinically, resulting in a failure to sufficiently reduce the dry weight. Cardiac biomarkers provide additional information but it is not clear whether this reflects fluid status or underlying structural organ damage. Intervention studies are now needed that show how this information is best used to improve patient outcomes, including meaningful end points such as hospitalization and survival.