This thesis presents an in-depth exploration of imperfections that can be found in self-catalysed III-V semiconductor nanowires. By utilising advanced electron microscopy techniques, the interface sharpness and defects at the atomic and macroscopic scale are analysed. It is found that a surprising variety and quantity of defect structures can exist in nanowire systems, and that they can in fact host some never-before-seen defect configurations. To probe how these defects are formed, conditions during nanowire growth can be emulated inside the microscope using the latest generation of in-situ...