This mini-review is dedicated to the use of nanotechnology for membranes, filters and sieves. With the advent of nanotechnology researchers have acquired an unprecedented freedom to sculpt device geometry almost down to the molecular scale. Such structures can now replace the gels, membranes and sieves of random pore structure that are commonly used in such separations. In this mini-review we want to highlight the impact that this development has had in the area of separation by filtering and sieving, where exciting developments are taking place. To do this we will first present the basic phenomena that determine separation in these devices, together with some historical background. Subsequently we will look at the micro- and nanomachined membranes, filters and sieves that have been manufactured in the past few years, and highlight advances resulting from the use of nanotechnology. In particular, the ability of nanotechnology to produce spatially anisotropic sieving structures suitable for continuous flow operation is seen as a new and exciting development. The invention of structures that sculpt hydrodynamic flow lines in order to perform continuous filtering is another important development. In general continuous flow operation is desirable in nanoscale systems since sampling (either of a detection signal or of the separated substances) can be time-integrated, thus improving the detection limit.