Iron (III) oxides are common constituents of geologic materials, they are products and by-products of many industrial processes, they are involved in biological processes, and they are the outcome of iron and steel corrosion. In many of these examples the iron oxides are — fortuitously or intentionally — of small particle size, and as a consequence difficult, if not impossible, to characterize by standard physicochemical techniques. 57Fe Mössbauer spectroscopy is suitable for this purpose because it can serve as a probe of the electric and magnetic conditions in the vicinity of iron nuclei in solid samples, no matter how the iron may be bound. Deviations of the magnetic properties of iron oxides of small particle size from those of their bulk counterparts lead to radical changes in the appearance of their Mössbauer spectra. Diverse models that have been put forward to account for such changes are discussed in this paper, including superparamagnetism, collective magnetic excitations, anomalous recoil-free fractions, superferromagnetism, spin canting and speromagnetism, reduced hyperfine field supertransfer, and Néel temperature reductions and distributions. Specific examples of microcrystalline iron (III) oxides and related minerals originating from different natural environments, resulting from technical processes, and being studied as planetary analogs are presented and discussed in the light of present-day knowledge on the properties of such materials.