Detonation processes have been observed in narrow, heavily confined, columns of solid explosive by a new microwave interferometric technique. The technique is described and a multiple-beam theory of fringe shape is given. The location, with respect to the detonation front, of the surface reflecting the microwaves is discussed. Detonation velocity as a function of distance along the column is derived from an oscilloscope display of the fringe pattern. The calculation of the detonation velocity requires a knowledge of the wavelength of the microwaves in the explosive. For this purpose the relative permittivities of a number of explosives are given as a function of their pressed density. The accuracy and applications of the method are discussed. Experiments on tetryl are described in which the technique is evaluated by observing the detonation velocity for a range of densities, and is applied to resolution of the velocity transient during growth to detonation. A simple theory of growth is used to estimate the reaction zone length (0$\cdot $4 mm) and the activation energy (2 kcal/mole) in the detonation of tetryl.