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Nanoindentation has become a rather mature tool to characterize and measure mechanical properties of materials on micro- and nanometer length scales. However, the different types of indentation size effects (ISE) hamper the assessment of true, i.e., scale independent, material properties by nanoindentation. In the present study, the mechanisms of the ISE in spherical indentation have been investigated and quantified. Furthermore, it has been shown that the determination of non-local crystal plasticity parameters from nanoindentation results is possible. The thus-parameterized non-local crystal plasticity describes the higher-order size effect occurring in spherical indentation correctly and offers the possibility to understand the mutual contributions of geometrically necessary dislocations (GND) and statistically stored dislocations (SSD) to the material response. Nanoindentation experiments have been performed into a single grain of an ARMCO iron specimen, where the load displacement curves have been recorded, and the post-indentation surface topologies have been characterized. Furthermore, transmission electron microscopy has been performed to analyse the plastically deformed volume beneath the indentation. By comparing experimental and numerical nanoindentation results with respect to the plastic zone and dislocation structures, we validate the non-local crystal plasticity model of indentation and increase our understanding of the underlying mechanisms of the ISE in spherical indentation.