A comprehensive statistical study of the phenomenology of aftershock sequences is made in this paper. The spatial distribution of aftershocks indicates that they are mainly crustal events; however, deeper sequences also take place. The analysis of the distribution of aftershocks in 15 sequences with respect to time and magnitude leads to the statistical confirmation of a set of phenomenological laws describing the process, namely, the time-frequency law of hyperbolic decay of aftershock activity with time, the magnitude stability law, and the exponential magnitude- frequency distribution. The hypotheses involved are checked. The grouping of data and the statistical methods employed are chosen according to some basic well·confirmed assumptions regarding the nature of the process.