An account is given of measurements which have been made of the variation of attenuation with frequency in a number of concentric-line filters. It is shown that the results are in rough agreement with formulae derived from transmission-line theory, though discrepancies arising from end-effects are by no means negligible. It is also shown that, for nearly all practical purposes, the cut-off frequency of a filter can be calculated from simple lumped-circuit theory.Constructional details are given for a number of low-pass and for one high-pass filter, and an appendix is included, giving Messrs. Proctor and Sloane's derivation of the transmission-line theory.