The Q-meter (or magnification factor meter) has become the recognized instrument for all kinds of r.f. impedance measurement. The resonant measuring circuit which forms the basis of such instruments may be excited from the oscillator source in different ways. In this paper, instruments have been classified into low-impedance and high-impedance injection types, according to the systems employed. The low-impedance resistance injection type has proved very satisfactory for use at frequencies below about 20 Mc/s. The various causes of error have been analysed and the difficulties of design for accuracy at the higher frequencies outlined. An instrument based on a novel principle of high-impedance injection has been developed, and it is possible to extend the range of accurate measurement to the region of 150 Mc/s. Slightly different technique is necessary when using the new principle, and illustrations of the use for various classes of measurement are given.A section of the paper is devoted to the definition ofQand the relationship between the quantities obtained on differing bases clarified. It is shown that the definitions ofQby frequency selectivity, by decrement, or by relation between energy storage and mean rate of dissipation are equivalent in the general case of a resonant system at a simple resonance. The essential difference between Q-value and magnification factor, as measured by a Q-meter, is discussed in detail.