the study of multiphase flow through porous media is undergoing intense development, mostly due to the recent introduction of new methods. after the profound changes induced by percolation in the eighties, attention is nowadays focused on the pore scale. the physical situation is complex and only recently have tools become available that allow significant progress to be made in the area. this volume on multiphase flow in porous media, which is also being published as a special issue of the journal transport in porous media, contains contributions on the lattice-boltzmann technique, the renormalization technique, and semi-phenomenological studies at the pore level. attention is mostly focused on two- and three-phase flows. these techniques are of tremendous importance for the numerous applications of multiphase flows in oil fields, unsaturated soils, the chemical industry, and environmental sciences.