The paper reports an investigation of the general problem of radio propagation influences on the performance of the azimuth channel of the conventional Instrument Landing System. The main text is an attempt to define the characteristics of I.L.S. signals that are more directly relevant to monitor design. To support the theoretical studies a localizer was installed at a disused airfield and the azimuthal guidance course was comprehensively monitored by means of signal probes erected in the required flight path. Geometricaloptical and diffraction theories were employed to assess the effects of possible re-radiating objects. The validity of the theoretical methods employed was checked by measurements at the disused airfield and at London Airport (Heathrow). The difficult task of assessing the probability of harmful interference between localizers operating in temperate climates on the same nominal frequency has been attempted by analysing the published data and augmenting it by recording anomalously propagated signals. Many flight trials have been undertaken in order to investigate the effect of overflying aircraft on the performance of the azimuth channel. Experimental records have been analysed in amplitude and frequency and compared with theoretical predictions. Results obtained with a ‘stationary’ landing aircraft have been used since the case of a real landing aircraft is at least no more critical in terms of the protection required, and an assessment has been made of the volume of air space which must be kept clear in order that there shall be a very low probability of interference from overflying aircraft.