GaAs nanowires grown by gas source molecular beam epitaxy for 3, 10, 30, and 40 min durations were studied by both scanning and transmission electron microscopy, providing a description of the time evolution of the nanowire morphology and structure. Tapered, "pencil-shaped" wires were observed in which a transformation of the sidewall orientation occurs from {(1) over bar 1 0 1} facets at the tip to {(2) over bar 1 1 0} facets at the base, providing evidence for a layer-by-layer radial growth model. The crystal structure of the nanowires, as well as the nature and frequency of stacking faults, was investigated. Local pseudo-periodicity of defects was observed in the vicinity of the wire base, while defect density decreased as the growth progressed. (C) 2007 Elsevier B.V. All rights reserved.