Abstract: Structural Biology is engaging ever larger assemblies of biomacromolecules either isolated, embedded in membranes or in whole cells. Thus, biophysical methods are needed that access these architectures on the critical nanometer length scale in these environments. Electron Paramagnetic Resonance provides several tools to precisely and reliably measuring such these distance in the nanometer range in particular a method called Pulsed Electron-Electron Double Resonance (PELDOR).1 In this presentation, it will be shown that PELDOR yields not only distances and distance distribution but also full information about label orientation,2 coupling mechanisms3 and that it can be used to count the monomers in aggregates.4 Examples will include covalently and non-covalently labelled duplex DNAs/RNAs, complex folds of RNAs and the 320 kDa membrane channel Wza5. 1. O. Schiemann, T.F. Prisner Quart. Rev. Biophys.2007, 40, 1. 2. O. Schiemann, P. Cekan, D. Margraf, T. F. Prisner, S.T. Sigurdsson, Angew. Chem. Int. Ed. Engl.2009, 121, 3342. 3. D Margraf, P. Cekan, T.F. Prisner, S.Th. Sigurdsson, O. Schiemann PCCP2009, 11, 6708. 4. B.E Bode, D. Margraf, J. Plackmeyer, G. Durner, T.F. Prisner, O. Schiemann J. Am. Chem. Soc.2007, 129, 6736. 5. G. Hagelueken, W.J. Ingledew, H. Huang, B. Petrovic-Stojanovska, C. Whitfield, H. ElMkami, O. Schiemann, J.H. Naismith Angew. Chem. Int. Ed.2009, 121, 2948.