AbstractA survey of the existing possibilities of molecular size and shape determination from rheological experiments is given. Because the hydrodynamic behaviour of particles depends on their size and shape at the conditions of experiment, one obtains by such methods just the dimensions as they are in the chosen solvent, at the temperature, ionization degree, pH, and ionic strength of the experiment. This fact is specially important in investigation of biological material, where one wishes to know the particle properties in natural surrounding or at least in a state as little denaturated as possible.With corpuscular particles, usually represented by spheroids, there are methods enough for obtaining all informations wanted even in the case that, due to swelling and selective adsorption of some solvent components, the particle density appreciably differs from that of dry particle. With linear particles, i.e. statistically coiled chain polymers, the main geometrical parameter is the gyration redius. It may be obtained by combination of translational motion and viscosity with proper consideration of solvent immobilisation inside the coil. The situation in the field, althought not yet elucidated enough, seems to converge rather rapidly to a complete understanding. The porous sphere model yields gyration radii which in nitrocellulose and to a lesser degree in polymethyl methacrylate very well agree with the values from light scattering. But hydrodynamic resistance coefficient of the monomer is highly underestimated. A more realistic approach, removing this discrepancy, is the neck‐lace model with hydrodynamic among all chain elements. For densily packet coils, e.g. in ideal solution (Θ‐point) and not too low molecular weight, one may assume complete solvent immobilisation so that gyration radii R0 are directly calculable from either limiting viscosity number or translational resistance coefficient. In systems not too far from Θ‐point the coil diameter can be calculated from R0 by series expension in powers of second virial coefficient and coil permeability obtained from rheological data. In good solvents, however, but the limiting case of complete solvent immobilisation is sufficiently well treated. Experiments on cellulose demonstrate that the permeability may reach rather high values as consequence of large coil expension. Due to the two parameters involved, the non‐Gaussian character and permeability of the coil, one cannot hope that, on the basis of the neck‐lace model, in such a case gyration radii may be derivable from rheological data only. On the contrary, by use of light scattering radii or eventually of those obtained by the porous sphere model one can test existing thermodynamic and hydrodynamic treatment of isolated polymer molecules in solution.