Hydraulic and lubrication systems have developed significantly in the past sixty years. The range of applications continue to expand. In modern systems, the application of proportional valves is a particularly important stimulus for the application of increasingly complicated and more accurate hydraulic drive systems. With the availability of more accurate production techniques that enable the possibilities of minimal tolerances, the operating pressures have more than doubled in the last 25 years. Pressures of up to 420 bar (42 MPa) are regarded as almost normal, and when it concerns clamping and stamping, pressure of 800 bar (80 MPa) are no longer unusual. The very narrow dynamic clearances of only a few micron (1 μ = 0.001 mm) between moving parts, mean that the fluid cleanliness of hydraulic and/or lubricating systems are afforded the highest priority. Better cleanliness levels, will improve system reliability. Some of the important points for this are: the cleanliness (= concentration of solid particles in function of their size), the water and air content, the additive composition, temperature, viscosity, acidity, and the presence of micro-organisms. This makes it necessary to choose the correct measurement equipment, such as the right technology for particle counters, as well as the components required for conditioning such as filters, coolers, heaters, dryers and sometimes also biocides. This Technical Instruction goes into the concept and technologies that play a role in keeping the fluid clean, dry and air free.