Membrane technology has emerged as a key component in a wide range of industrial applications. This growth is based on a consequent development of membrane materials, modules and operating modes. Due to economic aspects and their ability of forming a wide range of different structures, organic materials such as polyethersulfone (PES) or polyamide are used in a large majority of the technical processes. In order to provide large membrane areas, numerous module types have been developed. However, the modules have typical areas of application, with respect to their characteristic strengths and weaknesses in terms of production costs, packing density or fouling susceptibility. Capillary membranes are tubular shaped fibres, providing a comparatively large membrane area per module volume, low fabrication costs and a well control of concentration polarization. The multi-channel geometry that is already widespread in the fabrication of tubular ceramic membranes is able to increase the packing density and mechanical stability of polymeric capillary membranes. The aim of this work is the preparation, characterisation and optimisation of polyethersulfone multi-channel capillary membranes (MCM). Phase inversion of a homogeneous dope solution is the most important preparation process of polymer membranes. MCM fibres were fabricated by a modified wet-dry spinning procedure with a custom made seven needle spinneret. Various preparation parameters such as polymer concentration, molecular weight and content of a polymeric additive, length of the air gap and its humidity, temperature and composition of the precipitation medium or the post-treatment with an oxidising agent (sodium hypochlorite) were investigated. Effects on the fibre morphology and the separation performance (pure water flux and retention of macromolecules) were evaluated. The experimental results clearly illustrate that the spinning parameters are not independent from one another. Rheological analyses revealed a required minimum viscosity of the polymer solution in order to fabricate fibres with a sufficient wall thickness and mechanical strength. Water flux measurements and scanning electron microscopy (SEM) images showed the impact of water vapour in the air gap length. The porosity on the outer fibre shell was increased, which led to a reduction of the membrane resistance and a distinct increase of the permeation rate. Furthermore, the post-treatment procedure had a massive influence on the permeation performance of MCM. In contrast to fibres stored in pure water, a flux improvement by the factor 6-10 was observed for MCM post-treated with 5000 ppm sodium hypochlorite. The temperature of the internal and external coagulation medium (water) had a pronounced impact on the fibre structure, porosity and performance. An increase of the spinning temperature from 10 to 50 C led to a doubling of the permeation flux, whereas a decline of dextran retention from 95.4 to 60.7 % was obtained. The results were confirmed by SEM images, as overall porosity of the MCM fibre changed distinctly. Addition of ethanol, acetone or solvent to the internal and external precipitation medium led to a divergent development of the MCM permeability. Results revealed an increase of the membrane flux in terms of solvent addition to the coagulants. However, an excessive solvent content in the bore fluid led to a collapse of the multi-channel geometry. Due to their mechanical stability, fabricated PES multi-channel fibres appear to be suitable for low pressure (<10 bar) nanofiltration (NF) applications. NF membranes are generally fabricated as thin film composite membranes, characterised by a thin active polyamide layer located on the surface of a porous support material. The surfaces of the seven feed channels were coated by an interfacial polymerization process of two reactive monomers. First results showed that the preparation procedure of composite MCM fibres was generally practicable. The NF separation characteristics were improved in a subsequent optimisation process of the composite MCM. MgSO4 retention increased to 91.4 % by a variation of different production parameters such as drying duration, monomer ratio or the reaction time. The results of this work clearly indicate a potential for optimisation of PES multi-channel capillary fibres. On one hand, in terms of an enhancement of the separation performance, and on the other hand in terms of a further development of the polymeric MCM technology into the nanofiltration region.