The basis requirements to be satisfied by cab filters are discussed, and the behaviour of fibrous filters with respect to airflow resistance and the capture of particles by mechanical and electrostatic means are outlined. Optimisation of filter performance by means of pleating, to increase the area, is briefly reviewed, and the behaviour of both mechanical and electrically charged filters on loading is discussed. Finally, the requirements for tests of cab filter performance, both in the field and in the laboratory, are considered.