Modern technical devices in microelectronics, energy harvesting or display technology become ever smaller. Therefore, small-scale mechanical testing techniques are a rising field of research in order to satisfy the demand to investigate and understand the possibly changing underlying deformation mechanisms of materials used in these applications and thereby secure reliability of such devices. Depth sensing hardness testing, commonly designated as nanoindentation, can be such a method of choice and with advancing refinement of methodology a variety of materials properties can be extracted. In this PhD-thesis high temperature nanoindentation with a focus on development of high temperature methodology and thermal management was systematically accomplished. Thermal activation analyses were carried out to investigate the dominating deformation mechanisms on a variet of materials with a special focus on body-centered cubic metals, and therefore the influence of alloying elements and refinement of microstructure into the ultra-fine grained regime. It was found that in coarse grained W-Re alloys the alloying element Re mainly influence the low temperature plasticity by a reduction of the Peierls barrier, while at high temperatures dislocation-dislocation interaction was evident. Contrarily, in case of a reduced grain size to some hundreds of nanometers, the effect of Re on the mechanical properties primarily originates at high temperatures through grain boundary diffusion processes. By designing samples with custom grain boundary types on Ta2.5W, the role of misorientation of adjacent grains and the consequently changed interface diffusivity on the high-temperature plastic deformation behavior was highlighted. It was found that the temperature- and rate-dependence of the flow stress is strongly reduced for samples consisting mainly of low-angle grain boundaries, in contrast to a random high-angle character. Further the mechanical properties and deformation mechanisms occuring upon a phase transformation were examined in a previously non-existing way with micromechanical methods on pure metallic cobalt. It was found that a transformed crystal structure sharply leads to a change in plasticity. Cobalt specifically undergoes a development in the rate controlling deformation mechanism from friction stress controlled basal dislocation glide to dislocation cross slip as temperature increases. Upon phase transformation the process that controls plasticity was found to change again to obstacle-controlled dislocation glide. The present work clearly highlights the versatility of advanced high temperature nanoindentation methods as a tool for thermal activation analysis and determination of the dominating deformation mechanisms in a variety of materials. This will help in the development of future high performance materials with complex microstructures to tailor alloy design and meet their high requirements for specic materials properties.