Transport of fluid in and around nanometer-sized objects with at least one characteristic dimension below 100 nm renders possible phenomena that are not accessible at bigger length scales. This research field is termed nanofluidics and received its name only recently, but the roots in science and technology are broad. Nanofluidics has experienced a big growth during the last few years, confirmed by significant scientific and practical achievements. This review focuses on physical properties and operation mechanisms of the most common structures like nanometer-sizedopenings and nanowires in solution on a chip. As the surface-to-volume ratio increases with miniaturization it is big in nanochannels, resulting in surface charge governed transport, which allows ion separation and is described by a comprehensive electrokinetic theory. The charge-selectivity is most pronounced if the Debye screening length is comparable to the smallest dimension of the nanochannel cross-section, leading to a predominantly counterion containing nanometer-sized aperture. Such unique properties result in charge-based separation of biomolecules at the micronanochannel interface, and at this free energy barrier also size-based separation can be achieved when biomolecules and nanoconstrictions have similar dimensions. Furthermore, nanopores and nanowires bear interesting physics, and these structures demonstrate sensitive, label-free and realtime electrical detection of biomolecules, holding great promise in the life sciences. The purpose of this review is to describe physical mechanisms on the nanometer scale where new phenomena occur, in order to exploit these unique properties and realize integrated sample preparation and analysis systems.