We studied the influence of the substrate preparation and the growth conditions important to fabricate GaAs nanowires (NWs) with metal-organic vapor phase epitaxy. The growth parameters temperature, precursor partial pressures and growth duration were investigated. The definite choice of the V/III ratio enables NW length growth independent on the diameter. By investigating the temporal evolution of the GaAs-NW growth a diameter-dependent growth rate could be determined. Applying nanosphere lithography arranged GaAs-NW arrays were achieved. The NWs morphology and real structure was investigated using (high-resolution) transmission electron microscopy and selective area diffraction. The twin formation in GaAs NWs was investigated. A crystallographic model is presented. 2006 Elsevier B.V. All rights reserved. PACS: 61.46; 68.65.k; 78.67.n; 81.15.Gh; 81.16.c; 81.07.b Keywords: A1. Nanostructures; A3. Metal-organic vapor phase epitaxy; B2. Semiconducting gallium arsenide