Magnetite nanoparticles are ubiquitous in soils, sediments and mine wastes, and are also used in nanotechnological applications. We studied the influence of synthesis conditions on the size and shape of magnetite nanoparticles that were produced in inorganic co-precipitation processes. Variable parameters included the types of reagents and their concentrations, the pH, the temperature (from 9 to 90 °C) and the atmosphere (O2 and N2). The mineral phases, the morphologies, size and shape distributions of the resulting magnetite particles were characterised using transmission electron microscopy and X-ray powder diffraction. We managed to produce magnetite nanoparticles between ~11 and 120 nm and to control the mean sizes of the crystals within this range. The morphologies of the magnetite nanocrystals were also affected by the synthesis conditions and varied according to grain size. Crystals with diameters between 10 and 25 nm have irregular or round morphologies, whereas crystals larger than 50 nm are octahedral. Various nucleation and growth processes are invoked to explain the influence of synthesis conditions on the sizes and shapes of the product magnetite nanoparticles.