Abstract : Using data which correspond to 20.7 fb 1 integrated at the Y(4S) resonance by the BABAR experiment, we have fully reconstructed samples of 6967+/-95 B0/B0bar mesons and 7266 +/- 94 B+/B- mesons which are 90 % pure. This is the world's largest sample of fully reconstructed B mesons. This sample is a fundamental ingredient of time-dependent analyses which are an essential part of BABAR's physics programme. In these analyses, the temporal evolution of B mesons is studied to determine the physical quantities which are associated with CP violation, the frequency delta m of B0/B0bar oscillations and the lifetimes of B mesons. The first observation of CP violation in the B0/B0bar system has used our sample of neutral B mesons to disentangle the effects of this phenomenon from those of the imperfections of the experimental determination of the B mesons'flavor. Our sample constitutes the signal for the measurements of delta m and the lifetimes. The lifetime measurements were the primary goal of this thesis. We have obtained the following results: tau(B 0 ) = 1.546 +/- 0.032 (stat) +/- 0.022 (syst) ps . tau(B + ) = 1.673 +/- 0.032 (stat) +/- 0.023 (syst) ps . tau(B + )/tau(B 0 ) = 1.082 +/- 0.026 (stat) +/- 0.012 (syst) : The precision is approximately the same as that of the combined results from all previous experiments published in 2000. This is the first measurement of B meson lifetimes at the Y(4S) energy. We have established a new experimental technique which is optimised for this experimental environment. Our results are compatible with the combined results of the year 2000. The most recent theoretical predictions presented during the summer of 2001 are in excellent agreement with our measurements.