The success of a filtration process depends on the filter medium itself but also on the apparatus design and the process conditions. Here the discussion is focussed on woven media for cake filtration, but the principle findings seem to be transferable to vleeces depth filter media or membranes. Particle retention and the flow resistance are representing the main properties of a filter medium relevant for the filtration process. For an optimal filter operation further properties like mechanical strength, physical and chemical environment, wettability, tailormade fabrication and regenerability are of influence Particle retention and resistance of a filter medium are determined by pore size and structure and the interaction with the slurry. To characterize the pore size, the pore size distribution and the flow resistance different measuring techniques are available. The pore size can be measured often not directly and the question has to be answered how to correlate an indirectly measured „hydraulic pore diameter“ and the „true“ pore size. The bubble point respresents a physically definite pore size measuring method enabling the direct ... mehr correlation with a shpere of definite size. This had been validated by systematic investigations of different pore structures and in comparison to other measuring methods. The flow resistance of a filter medium is characterized not adequate in the case of practical filtration by measuring only with particle-free fluid. How can be demonstrated, it must be measured by using the real suspension to be separated to get informations of practical relevance. Finally process relevant phenomena like filtrate pollution by not separated fine particles or detachment of a filter cake from the filter cloth are influenced not only by the filter medium but also by design parameters of the apparatus and operational parameters of the process.