The energy cost for various ciliary filter feeders shows that useful pump work constitutes 0.3–1.1% of the total metabolic expenditure. The ‘water processing potential’ (liters of water pumped per milliliter O2 consumed by the animal) is a useful tool for characterizing filter feeding and adaptation to the environment. The six types of ciliary‐capture mechanisms (collar sieving, cirral trapping, ciliary sieving, ciliary downstream collecting, ciliary upstream collecting, mucus‐net sieving) are reviewed as well as the aerosol/hydrosol filtration theory. A brief overview of fluid mechanical principles and tools for studying ciliary functions is given.