Fluidization occurs when an upward flow of fluid passing through a bed of particles becomes sufficient to support them against gravity. The pressure drop across the bed then equals the bed weight and the particles are free to move. At this point (the point of incipient or minimum fluidization) the bed is said to be fluidized. With good design, mixing in fluidized beds is very rapid, the bed is nearly isothermal and high rates of heat and mass transfer can be attained. The applications of fluidized beds that are of relevance here are (1) coating, e.g. of pharmaceutical products, particularly pellets and tablets [17] and (2) filtration, where fine (0.5–10 μm) particles can be captured on larger fluidized carrier particles e.g. to remove environmentally-damaging and health-threatening particles from dirty gas streams passed upwards through a fluidized bed [18]. By injecting a RESS jet directly into a fluidized bed of carrier excipient particles, these two concepts can be combined to allow nanoparticulate coating of active material onto excipient particles. The immediate collection of nanoparticles from a RESS process, close to their point of origin, would prevent their further growth and agglomeration, so forming a product which captures and retains the efficacy and special features of the nanometer size in an easily handleable form. The capture of particles in this way also provides a means of alleviating concerns relating to the inappropriate escape of very small particles into the environment. In contrast to conventional fluidized bed coating, this strategy also avoids the use of liquids.