Planar and three-dimensional multi-electrode systems with dimensions of 2 - 40 mu m were fabricated by IC technology and used for trapping and aggregation of microparticles. To achieve negative dielectrophoresis (repelling forces) in aqueous solution, radiofrequency (RF) electric fields were used. Experimentally, particles down to 100 nm in diameter were enriched and trapped as aggregates in field cages and dielectrophoretic microfilters and observed using confocal fluorimetry. Theoretically, single particles with an effective diameter down to about 35 nm should be trappable in micron field cages. Due to the unavoidable Ohmic heating, RF electric fields can induce liquid streaming in extremely small channels (12 mu m in height). This can be used for pumping and particle enrichment but it enhances Brownian motion and counteracts dielectrophoretic trapping. Combining Brownian motion with ratchet-like dielectrophoretic forces enables the creation of Brownian pumps that could be used as sensitive separation.