The proposal describes the initiation of a collaboration between the groups of Edward Reijerse (MPI-Mülheim), Anton Savitsky (MPI-Mülheim) and the group of Dieter Suter (University of Dortmund) to boost the sensitivity of Electron Paramagnetic Resonance (EPR) spectroscopy for very small samples. The expertise of the “Suter group” in the design and construction of planar micro-resonators is combined with the experience at the MPI to design and construct EPR probeheads for “special applications”, i.e. experiments for which commercial probeheads are not suited. The new microresonators will be used to investigate very small samples, in particular protein micro-crystals (typical dimensions: 0.2×0.2×0.4 mm3) of hydrogenases and photosystems, at X- and Q-band frequencies (i.e. 9.5 and 34 GHz) using CW and pulsed EPR. A prototype 244 GHz probe-head at the MPI will be further developed so that the same micro-crystals can also be studied at very high microwave frequency. Step by step the resonators will be expanded to allow double resonance measurements (ENDOR) and light access. Several “spin-off” developments in this project are anticipated: 1) An accurate EPR Gaussmeter based on the micro-resonator design. 2) The integration of a cryogenic preamplifier and further electronics in the micro-probehead to boost its sensitivity. 3) A micro-strip resonator for the detection of paramagnetic monolayers, e.g. on a gold electrode. Since the design and production of micro-resonators is also relevant for other applicants in the priority program this task will be described in a “central application” by the group of Dieter Suter. The developed micro-resonator probeheads will open completely new possibilities to study very small proteins crystals which were inaccessible to EPR up to now. The micro-resonator based EPR Gaussmeter will solve the long standing problem of accurate and dynamic field monitoring in superconducting magnets.