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Magnetic nanoparticles (MNP) as markers for the specific analysis of biomolecules have the advantage that they are stable, non-toxic and that they can be used in opaque media or tissue. Magnetorelaxometry is based on the fact that a MNP relaxes after being aligned by a magnetic field pulse dependent on its size and on whether it is mobil or immobilized. The keypoint is that MNPs bound to specific biological targets can be distinguished from unbound ones by their different relaxation times and relaxation behaviour without the need to wash away unbound markers. In the originally introduced magnetic relaxation immuno assay (MARIA) molecules labelled with MNPs and bound to a molecular structure bound to a solid phase are immobilized, thus suppressing Brownian relaxation. Unbound MNP-labelled molecules show Brownian relaxation, supposed that the diameter of the MNPs exceeds a certain critical value. The current activities are focussed on the analysis of biomolecules, such as bacteria, viruses and even certain proteins, in solution, i.e., without the need to immobilize them. This paper gives an overview about the research activities in the field of magnetorelaxometry and addresses the requirements on MNPs.