This paper describes possibilities to separate particles with difficult dust proper-ties from gases. Difficult dust properties are related to extreme values of particle size and shape and to the flowability, the adhesion properties or the reactivity of the particles. Special emphasis is given to submicron particles. In cyclones, conductive particles such as diesel soot can be removed by means of additional electrostatic forces. Experimental investigations into wet tubular electrostatic precipitators show that the measured separation efficiencies are much higher than theoretically anticipated. This result is explained by higher particle charges than predicted by the existing charging models. For small flow rates where electrostatic precipitators are economically not feasible, a new type of wet scrubber may be an alternative. The critical issue of surface filters is the adhesion of the dust cake at the surface of the filter medium. Regeneration of the dust cake as well as trends for the separation efficiency can be determined by small coupon testers which can be used for lab investigations and for field tests. Surface filters are widely used in industry for the separation of nanoparticles. Even extremely sticky particles such as tar particles can be removed by surface filters if a precoat layer protects the filter medium. It is shown that by looking at the physical fundamentals of particle separation, new and innovative solutions can be discovered. Guidelines for the separation of particles with difficult dust properties are given.