The physics of the two types of resonances characterizing the dynamical structure of the asteroidal belt are presented. In the case of mean motion resonances, the frequency of Jupiter´s rotation about the Sun determines the various resonance frequencies. For secular resonances, the resonance frequencies corxespond to eigenfrequencies of the system of differential equations descríbing the motions of nodal and apsidal lines of planetary orbits. Both
types of resonances are related to gaps in the asteroidal belt. The computing theories for the formation of these gaps are reviewed. The secular resonances gained considerable interest because of their alleged role as source of Earth-crossing objects like Apollo asteroids and meteorites. Recent numerical model calculations on secular resonances are reviewed under this aspect.