Abstract Recent observations made on the ground and by satellites have revealed a variety of VLF emissions. In the first part of this paper, the observational results are reviewed. The intensity diurnal and seasonal variations are summarized paying particular attention to chorus and hiss, and characteristics of these two kinds of emissions are compared with other geomagnetic phenomena. Apparent differences are found between these two kinds of emissions suggesting that the generating mechanisms are greatly different. Characteristics of artificially stimulated emissions (ASE), one of which is offset phenomenon, and those of quasiperiodic emissions are described in detail. In the second part, theoretical interpretations for the mechanism of VLF emissions are surveyed. Generally, there are three main theoretical categories; non-coherent radiations, coherent beam wave interactions and radio emissions from a plasma wave. The first are radiations from drifting single particles, the intensity of which is, however, insufficient in comparison with that of the observed emissions. The second deals with instabilities for whistler wave mode which may arise at the gyroresonance condition in a plasma beam system and may be the most promising mechanisms for the generation of intense VLF emissions. The third consists of wide band radioe missions excited by hydromagnetic shock waves, which may be a mechanism of hiss. The frequency versus time spectra of each kind of VLF emissions are discussed, which are the crucial properties for classification of the emissions. It is concluded that the theories of beam wave interaction are not yet complete to the point of interpretation for every fine spectral structure. Theoretical interpretation for the offset phenomenon of ASE is proposed by the present author, which is based on cyclotron instability by an electron beam. The quasiperiodic emissions may be understood as being produced by a sequence of offset effects. Energies of the beam particles (e.g. electrons or protons) required for gyroresonance and the frequency of emissions to be generated are calculated for geocentric distances, under appropriate assumptions. Then the energies of the particles needed for the generation of chorus and hiss are evaluated.