The estimation of body fluids is a useful and common practice in the status assessment of disease mechanisms and treatments. Electrical bioimpedance spectroscopy (EBIS) methods are non-invasive, inexpensive, and efficient alternatives for the estimation of body fluids. However, these methods are indirect, and their robustness and validity are unclear. Regarding the recording of measurements, a controversy developed regarding a spectrum deviation in the impedance plane, which is caused by capacitive leakage. This deviation is frequently compensated for by the extended Cole model, which lacks a theoretical basis; however, there is no method published to estimate the parameters. In this thesis, a simplified model to correct the deviation was proposed and tested. The model consists of an equivalent capacitance in parallel with the load. Subsequently, two other measurement artefacts were considered. Both artefacts were frequently disregarded with regard to total body and segmental EBIS measurements as their influence is insignificant with suitable skin-electrode contact. However, this case is not always valid, particularly from a textile-enabled measurement system perspective. In the estimation of body fluids, EBIS data are fitted to a model to obtain resistances at low and high frequencies. These resistances can be related to body fluid volumes. In order to minimise the influence of all three artefacts on the estimation of body fluids and improve the robustness and suitability of the model fitting the different domains of immittance were used and tested. The conductance in a reduced frequency spectrum was proposed as the most robust domain against the artefacts considered. The robustness and accuracy of the method did not increase, even though resistances at low and high frequencies can be robustly estimated against measurement artefacts. Thus, there is likely error in the relation between the resistances and volumes. Based on a theoretical analysis, state of the art methods were reviewed and their limitations were identified. New methods were also proposed. All methods were tested using a clinical database of patients involved in growth hormone replacement therapy. The results indicated EBIS are accurate methods to estimate body fluids, however they have robustness limits. It is hypothesized that those limits in extra-cellular fluid are primarily due to anisotropy, in total body fluid they are primarily due to the uncertainty ρi, and errors in intra-cellular fluid are primarily due to the addition of errors in extracellular and total body fluid. Currently, these errors cannot be prevented or minimised. Thus, the limitations for robustness must be predicted prior to applying EBIS to estimate body fluids.