Taper junctions of modular hip prostheses are susceptible to fretting and crevice corrosion. Prevalence and significance increase for cobalt-chromium heads assembled on titanium-alloy stems. Retrieval and in-vitro studies have identified micromotion between the taper components to accelerate the corrosion process. The aim of this study was to identify the most critical factors contributing to increased micromotion, which is most likely influenced by design-, patient- and surgeon-related aspects. Micromotion between head and stem taper surfaces was measured for different taper surface topographies and load orientations. Consecutive visual images were recorded through windows in the head component. By image matching analysis the local micromotions at the taper junction between head and stem tapers were determined. To extend the findings to taper regions not visible through the windows, finite element models were generated. The models were further utilized to investigate the influence of head length, taper angle difference and assembly force on micromotion. Significantly higher micromotion (+20%) was found under varus loading (7.1 µm) in comparison to valgus loading (5.9 µm). Smooth and microgrooved stem tapers exhibited equal amounts of micromotion. The numerical model revealed head tilting and recurring taper contact changes in terms of cyclic engagement/disengagement during the loading sequences. Especially long heads (+240%) and low assembly forces (+53%) were found to substantially increase micromotion (from 2.7 µm to 9.3 µm and from 4.1 µm to 8.8 µm, respectively). This study accentuates the susceptibility of taper junctions to a variety of factors, which need to be appreciated in preoperative planning and surgical procedure to reduce the amount of micromotion and such minimize the risk of critical corrosion.