Total body bioelectrical impedance is a measure for total body water. From total body water the fat free mass and, by difference with body weight, the fat mass (body fat percent) can be calculated. Prediction formulas for body fat percent based on impedance assume a constant hydration of the fat free mass, an assumption that might be violated in individuals, thus resulting in biased individual predictions. The electrical properties of the human body depend on water distribution between extra- and intra-cellular space and also on geometrical water distribution. In addition, body build factors also influence total or segmental impedance values. As body build and water distribution can differ between populations, impedance formulas aiming to quantify water compartments or body fat percent should also be population specific. In addition other factors like body position, skin and/or body temperature and osmotic values of the body fluids have an impact on the measured impedance values. This, together with likely disturbances in body water distribution in patients, makes the quantitative use of impedance in clinical practice difficult and prone to misinterpretations. For the prediction of body fat percent in populations, the method can be suitable. But, as there are differences in body build between (ethnic) population groups, for example in relative leg length and in slenderness, it is likely that that prediction formulas have different bias across ethnic groups. This means that for meaningful comparisons between population groups, formulas have to be validated in these groups.