The spectrum of filtration separations runs from the millimeter scale (beach sand and activated carbon particles) using coarse filters, to the angstrom scale (metal ions and gas molecules) using reverse osmosis or gas separation membranes. Between there are microfiltration (bacteria and emulsions), ultrafiltration (proteins, viruses, and colloids), and nanofiltration (sugars, herbicides, small organic molecules). In the analytical environment it is likely that membrane and filtration systems involving this entire spectrum will be used to provide high purity water, gases, reagents, and even special functions within instruments. In addition to these applications, membranes and filters will often also be used in sample preparation and perhaps initial characterization. In this entry, we will discuss the principles, materials, and devices used to accomplish coarse, micro- and ultrafiltration on the laboratory scale.