The development of coupled-transmission-line theory and applications has extended the bandwidth of certain microwave components to one decade or more. A review of the theory is presented in the most simple general form, which enables the effects of tolerances and perturbations to be calculated. A survey of available coupled-line configurations is presented, including a new printed re-entrant type, with, in some instances, more accurate formulas than those available in the literature. The theoretical aspects of this work are substantially complete, but there are practical difficulties associated with obtaining good directivity and higher-frequency devices.