Reasons for the ozone layer depletion have been investigated by means of a statistical analysis of vertical ozone profile variations at nine stations of the Global Ozone Network (including four Canadian, American, three Japanese, and German stations) and time series analysis of total ozone at twenty-five stations, including the longest time series started at Arosa, Switzerland, in 1926. Significant positive trends of the height (at seven stations) and width (at five stations) of the ozone maximum (for the significance level P = 0.95) have been revealed; at the other stations the trends are shown to be insignificant (but positive too). An analysis of the most significant forty-one total ozone anomalies indicates that practically always the ozone depletion becomes most pronounced below the ozone maximum. The widespread model description of the total ozone time series by "hockey stick" is not statistically optimal. The dynamic mechanism of the ozone depletion should dominate over the anthropogenic one. The existing observational data are insufficient for adequate description of the long-term ozone depletion tendencies, to say nothing of their reasons (or the data are too noisy). The adequate temporal description of the total ozone should consider the global climate changes.